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404

There's nothing here.
+ Take me home. +
+ + + diff --git a/ES6+/array.html b/ES6+/array.html new file mode 100644 index 0000000..aa81683 --- /dev/null +++ b/ES6+/array.html @@ -0,0 +1,282 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 扩展运算符（spread）

扩展运算符（spread）是三个点（...）。它好比 rest 参数的逆运算，将一个数组转为用逗号分隔的参数序列。

console.log(...[1, 2, 3])
+// 1 2 3
+
+console.log(1, ...[2, 3, 4], 5)
+// 1 2 3 4 5
+
+[...document.querySelectorAll('div')]
+// [<div>, <div>, <div>]
+
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# 静态方法

  • Array.of: 用于将一组值，转换为数组。
  • Array.from: 用于将两类对象转为真正的数组：类似数组的对象（array-like object）和可遍历（iterable）的对象（包括 ES6 新增的数据结构 Set 和 Map）。

# Array.of

# Array.of & Array

Array()传入数字会生成空的长度为 3 的数组。Array.of()不仅可以将某个数值变为数组，也可以将一组值变为数组。我们通过下面的代码看看他们的区别

const a = Array(3)
+a.length // 3
+a[0] // undefined
+
+const b = Array.of(3)
+b.length // 1
+b[0] // 3
+
+const c = Array.of(1, 2, 3)
+c.length // 3
+c // [1,2,3]
+
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不难看出，Array.of 完美弥补了 Array()的不足，能快速生成你想要的数组。

let a = Array() // []
+let b = Array(3) // [, , ,]
+let c = Array(3, 11, 8) // [3, 11, 8]
+
+Array.of() // []
+Array.of(undefined) // [undefined]
+Array.of(1) // [1]
+Array.of(1, 2) // [1, 2]
+
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# Array.of & Array.prototype.slice.call

Array.of 和下面 of 的方法处理类似。

function of() {
+  return Array.prototype.slice.call(arguments)
+}
+
+// 也可以这么写
+function ArrayOf() {
+  return [].slice.call(arguments)
+}
+
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但是你能直接用Array.prototype.slice.call直接替换，他们处理参数有很大区别

Array.prototype.slice.call([1, 2, 3]) // [1, 2, 3]
+Array.of(1, 2, 3) // [1, 2, 3]
+
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# Array.of & new Array

我们先看看new Array如何生成数组，

new Array()
+// []
+new Array(undefined)
+// [undefined]
+new Array(1)
+// [undefined x 1]
+new Array(3)
+// [undefined x 3]
+new Array(1, 2)
+// [1, 2]
+new Array(-1)
+// RangeError: Invalid array length
+
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Array.of很好的解决了特殊长度-1 的问题。

Array.of()
+// []
+Array.of(undefined)
+// [undefined]
+Array.of(1)
+// [1]
+Array.of(3)
+// [3]
+Array.of(1, 2)
+// [1, 2]
+Array.of(-1)
+// [-1]
+
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# Array.from

Array.from()方法用于将两类对象转为真正的数组：类似数组的对象（array-like object）和可遍历（iterable）的对象（包括 ES6 新增的数据结构 Set 和 Map）。

JavaScript 中的“类数组对象”是一个具有长度属性的对象，特别是整数值为零或更高的对象。
+在 ES6 之前我们需要这样转化成数组

// array-like object类似数组的对象，含有length属性
+const arrLike = {
+  length: 3,
+  0: 'foo',
+  1: 'bar',
+}
+
+const arr = Array.prototype.slice.call(arrLike)
+console.log(arr) // ['foo', 'bar', empty] 虽然浏览器显示的empty,但实际是没有第三个值的，长度为3，第三个值打印出来会是undefined
+
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slice()可以用作复制数组

const arrLike = {
+  length: 3,
+  0: 'foo',
+  1: 'bar',
+}
+
+const arr = Array.prototype.slice.call(arrLike)
+const arr2 = arr.slice()
+arr2.push(2)
+console.log(arr, arr2) // ['foo', 'bar', empty] ['foo', 'bar', empty, 2]
+
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而用 ES6 的 Array.from 来操作转换数组和复制的操作，更加优雅简洁。

const arrLike = {
+  length: 3,
+  0: 'foo',
+  1: 'bar',
+}
+
+const arr = Array.from(arrLike)
+const arr2 = Array.from(arr)
+arr2.push(2)
+console.log(arr, arr2) // ['foo', 'bar', undefined] ['foo', 'bar', undefined, 2]
+
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Array.from 可以传三个参数，第一个参数可以是类数组（）或可遍历对象（）。第二个参数是函数。如果 map()函数里面用到了 this 关键字，还可以传入 Array.from()的第三个参数，用来绑定 this。

Array.from(arrayLike)
+Array.from(arrayLike, mapFn)
+Array.from(arrayLike, mapFn, thisArg)
+
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第二个参数是函数，类似于操作 map 遍历操作内部数据。

Array.from({ length: 3 }) // [ undefined, undefined, undefined ]
+
+Array.from(arrayLike, (x) => x * x)
+// 等同于
+Array.from(arrayLike).map((x) => x * x)
+
+Array.from([1, 2, 3], (x) => x * x)
+// [1, 4, 9]
+
+Array.from([1, , 2, , 3], (n) => n || 0)
+// [1, 0, 2, 0, 3]
+
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只要是部署了 Iterator 接口的数据结构，Array.from()都能将其转为数组。

Array.from('hello')
+// ['h', 'e', 'l', 'l', 'o']
+
+let namesSet = new Set(['a', 'b'])
+Array.from(namesSet) // ['a', 'b']
+
+let nameMap = new Map([
+  [1, 'one'],
+  [2, 'two'],
+  [3, 'three'],
+])
+console.log(Array.from(nameMap)) //   [[1,'one'],[2,'two'],[3,'three']]
+
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利用Array.from创建四个空值

const a = Array(4) // four empty slots!
+
+const b = Array.apply(null, { length: 4 }) // four `undefined` values
+const c = Array.from({ length: 4 }) // four `undefined` values
+console.log(a, b, c) // [empty × 4] [undefined, undefined, undefined, undefined] [undefined, undefined, undefined, undefined]
+
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由于空位的处理规则非常不统一，所以建议避免出现空位。尽量让空位内有值 undefined。

Array.from({ length: 4 }) // [undefined, undefined, undefined, undefined]
+Array.from({ length: 4 }, (v) => (v = 1)) // [1, 1, 1, 1]
+
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# copyWithin

数组实例的 copyWithin()方法，在当前数组内部，将指定位置的成员复制到其他位置（会覆盖原有成员），然后返回当前数组。也就是说，使用这个方法，会修改当前数组。

Array.prototype.copyWithin(target, (start = 0), (end = this.length))
+
1

示例 1：

var items = [1, 2, 3, , , , , , , ,]
+items.copyWithin(6, 0, 3) //  [1, 2, 3, empty × 3, 1, 2, 3, empty]
+
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示例 2：

var items = [1, 2, 3, , , , , , , ,]
+items.copyWithin(6, 1, 3) // [1, 2, 3, empty × 3, 2, 3, empty × 2]
+
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示例 3：

var items = [1, 2, 3, , , , , , , ,]
+items.copyWithin(6, 2, 3) // [1, 2, 3, empty × 3, 3, empty × 3]
+
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当参数为负值，如何操作的。

[1, 2, 3, ,,,,,,,].copyWithin(-3, 1)
+// <- [1, 2, 3, undefined x 4, 2, 3, undefined x 1]
+[1, 2, 3, ,,,,,,,].copyWithin(-6, -8)
+// <- [1, 2, 3, undefined x 1, 3, undefined x 5]
+[1, 2, 3, ,,,,,,,].copyWithin(-3, 1, 2)
+// <- [1, 2, 3, undefined x 4, 2, undefined x 2]
+
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# fill

fill() 方法用一个固定值填充一个数组中从起始索引（默认为 0）到终止索引（默认为 array.length）内的全部元素。它返回修改后的数组。

console.log(['a', 'b', 'c'].fill(0)) // [0, 0, 0]
+console.log(new Array(3).fill(0)) // [0, 0, 0]
+console.log(['a', 'b', 'c', , ,].fill(0, 2)) // ['a', 'b', 0, 0, 0]
+console.log(new Array(5).fill(0, 0, 3)) // [0, 0, 0, empty × 2]
+console.log(new Array(3).fill({})) // [{}, {}, {}]
+console.log(new Array(3).fill(function foo() {})) // [function foo () {}, function foo () {}, function foo () {}]
+
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语法：

fill(value)
+fill(value, start)
+fill(value, start, end)
+
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注意，如果填充的类型为对象，那么被赋值的是同一个内存地址的对象，而不是深拷贝对象。

let arr1 = new Array(3).fill({ name: 'Mike' })
+arr1[0].name = 'Ben'
+console.log(arr1)
+// [{name: "Ben"}, {name: "Ben"}, {name: "Ben"}]
+
+let arr2 = new Array(3).fill([])
+arr2[0].push(5)
+console.log(arr2)
+// [[5], [5], [5]]
+
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# find

find() 方法返回数组中满足提供的测试函数的第一个元素的值。否则返回 undefined。

如果需要在数组中找到对应元素的索引，请使用 findIndex()。
+如果需要查找某个值的索引，请使用 Array.prototype.indexOf()。（它类似于 findIndex()，但只是检查每个元素是否与值相等，而不是使用测试函数。）
+如果需要查找数组中是否存在某个值，请使用 Array.prototype.includes()。同样，它检查每个元素是否与值相等，而不是使用测试函数。
+如果需要查找是否有元素满足所提供的测试函数，请使用 Array.prototype.some()。

语法：

find(callbackFn)
+find(callbackFn, thisArg)
+
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示例：

let arr1 = [1, 2, 3, 4, 5].find((item) => item > 2)
+// <- 3
+let arr2 = [1, 2, 3, 4, 5].find((item, i) => i === 3)
+// <- 4
+let arr3 = [1, 2, 3, 4, 5].find((item) => item === Infinity)
+// <- undefined
+console.log(arr1, arr2, arr3)
+
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非数组种使用：

const arrayLike = {
+  length: 3,
+  0: 2,
+  1: 7.3,
+  2: 4,
+}
+console.log(Array.prototype.find.call(arrayLike, (x) => !Number.isInteger(x))) // 7.3
+
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# findIndex

findIndex() 方法返回数组中满足提供的测试函数的第一个元素的索引。若没有找到对应元素则返回 -1。

console.log(
+  [1, 5, 10, 15].findIndex(function (value, index, arr) {
+    return value > 9
+  })
+) // 2
+console.log([1, , 3].findIndex((x) => x === undefined)) // 1
+const arrayLike = {
+  length: 3,
+  0: 2,
+  1: 7.3,
+  2: 4,
+}
+console.log(
+  Array.prototype.findIndex.call(arrayLike, (x) => !Number.isInteger(x))
+) // 1
+
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# 迭代器协议相关方法

keys：keys() 方法返回一个新的数组迭代器 (en-US)对象，其中包含数组中每个索引的键。
+values：values() 方法返回一个新的数组迭代器 (en-US)对象，该对象迭代数组中每个元素的值。
+entries：entries() 方法返回一个新的数组迭代器 (en-US)对象，该对象包含数组中每个索引的键/值对。 +[Symbol.iterator]：Array 实例的 @@iterator 方法实现了迭代协议，允许数组被大多数期望可迭代对象的语法所使用，例如展开语法和 for...of 循环。它返回一个数组迭代器对象 (en-US)，该对象会产生数组中每个索引的值。

const array1 = ['a', 'b', 'c']
+const iteratorK = array1.keys()
+const iteratorV = array1.values()
+
+for (const key of iteratorK) {
+  console.log(key)
+}
+
+// 0
+// 1
+// 2
+
+for (const key of iteratorV) {
+  console.log(key)
+}
+
+// 'a'
+// 'b'
+// 'c'
+
+for (const [index, element] of array1.entries()) {
+  console.log(index, element)
+}
+
+// 0 'a'
+// 1 'b'
+// 2 'c'
+
+const iteratorI = array1[Symbol.iterator]()
+for (const value of iteratorI) {
+  console.log(value)
+}
+
+// 'a'
+// 'b'
+// 'c'
+
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Last Updated: 2024-4-2 11:29:35 ├F10: PM┤

+ ← + + 前言 + + → +

+ + + diff --git a/ES6+/arrayES6+.html b/ES6+/arrayES6+.html new file mode 100644 index 0000000..29b3b05 --- /dev/null +++ b/ES6+/arrayES6+.html @@ -0,0 +1,91 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# includes

includes() 方法用来判断一个数组是否包含一个指定的值，根据情况，如果包含则返回 true，否则返回 false。

console.log([1, 2, 3].includes(2)) // true
+console.log([1, 2, 3].includes(4)) // false
+console.log([1, 2, NaN].includes(NaN)) // true
+
+// 第二个参数表示搜索的起始位置，默认为0。如果为负数，则表示倒数的位置，如果这时大于数组的长度则会重置为从0开始
+console.log([1, 2, 3].includes(2, -4)) // true
+console.log([1, 2, 3].includes(2, -3)) // true
+console.log([1, 2, 3].includes(2, -2)) // true
+console.log([1, 2, 3].includes(2, -1)) // false
+console.log([1, 2, 3].includes(2, 0)) // true
+console.log([1, 2, 3].includes(2, 1)) // true
+console.log([1, 2, 3].includes(2, 2)) // false
+console.log([1, 2, 3].includes(2, 3)) // false
+console.log([1, 2, 3].includes(2, 4)) // false
+
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# flat

flat() 方法创建一个新的数组，并根据指定深度递归地将所有子数组元素拼接到新的数组中。

# 语法

flat()
+flat(depth)
+
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depth 可选 +指定要提取嵌套数组的结构深度，默认值为 1。

# 案例

const arr1 = [0, 1, 2, [3, 4]]
+
+console.log(arr1.flat())
+// [0, 1, 2, 3, 4]
+
+const arr2 = [0, 1, [2, [3, [4, 5]]]]
+
+console.log(arr2.flat())
+// [0, 1, 2, [3, [4, 5]]]
+
+console.log(arr2.flat(2))
+// [0, 1, 2, 3, [4, 5]]
+
+console.log(arr2.flat(Infinity))
+// [0, 1, 2, 3, 4, 5]
+console.log(arr2.flat(3))
+// [0, 1, 2, 3, 4, 5]
+
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# flatMap

flatMap() 方法对数组中的每个元素应用给定的回调函数，然后将结果展开一级，返回一个新数组。它等价于在调用 map() 方法后再调用深度为 1 的 flat() 方法（arr.map(...args).flat()），但比分别调用这两个方法稍微更高效一些。

const arr1 = [1, 2, 1]
+let result1 = arr1.flatMap((num) => num * 2)
+console.log(arr1, result1) // [1, 2, 1] [2, 4, 2]
+
+const arr2 = [1, 2, 1]
+let result2 = arr2.flatMap((num) => [num * 2])
+console.log(arr2, result2) // [1, 2, 1] [2, 4, 2]
+
+const arr3 = [1, 2, 1]
+let result3 = arr2.flatMap((num) => [[num * 2]])
+console.log(arr3, result3) // [1, 2, 1] [[2], [4], [2]]
+
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Last Updated: 2024-4-2 11:31:49 ├F10: PM┤
+ + + diff --git a/ES6+/destructuring.html b/ES6+/destructuring.html new file mode 100644 index 0000000..7051515 --- /dev/null +++ b/ES6+/destructuring.html @@ -0,0 +1,193 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 1.概览

解构只是意味着将复杂的结构分解成更简单的部分。在 JavaScript 中，这种复杂的结构通常是一个对象或一个数组。使用解构语法，您可以从数组和对象中提取更小的片段。解构语法可用于变量声明或变量赋值。您还可以使用嵌套解构语法来处理嵌套结构。

// old
+const student = {
+  name: 'John Doe',
+  age: 16,
+  scores: {
+    maths: 74,
+    english: 63,
+    science: 85,
+  },
+}
+
+function displaySummary(student) {
+  console.log('Hello, ' + student.name)
+  console.log('Your Maths score is ' + (student.scores.maths || 0))
+  console.log('Your English score is ' + (student.scores.english || 0))
+  console.log('Your Science score is ' + (student.scores.science || 0))
+}
+
+displaySummary(student)
+
+// Hello, John Doe
+// Your Maths score is 74
+// Your English score is 63
+// Your Science score is 85
+
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用 ES6 结构赋值，我们可以这么写

// ES6
+function displaySummary({
+  name,
+  scores: { maths = 0, english = 0, science = 0 },
+}) {
+  console.log('Hello, ' + name)
+  console.log('Your Maths score is ' + maths)
+  console.log('Your English score is ' + english)
+  console.log('Your Science score is ' + science)
+}
+
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# 2.对象解构

对象解构构像是：用变量声明属性，变量值即为属性值，简单来说，就是提取对象中的属性。

const student = {
+  firstname: 'Glad',
+  lastname: 'Chinda',
+  country: 'Nigeria',
+}
+
+// Object Destructuring
+const { firstname, lastname, country } = student
+
+console.log(firstname, lastname, country) // Glad Chinda Nigeria
+
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利用解构给变量重新赋值

// Initialize local variables
+let country = 'Canada'
+let firstname = 'John'
+let lastname = 'Doe'
+
+const student = {
+  firstname: 'Glad',
+  lastname: 'Chinda',
+  country: 'Nigeria',
+}
+
+// Reassign firstname and lastname using destructuring
+// Enclose in a pair of parentheses, since this is an assignment expression
+;({ firstname, lastname } = student)
+
+// country remains unchanged (Canada)
+console.log(firstname, lastname, country) // Glad Chinda Canada
+
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上面代码，用解构赋值的方式重新给 firstname, lastname 变量赋值，所以后面打印的时候，firstname, lastname 显示是 student 内部的 firstname, lastname。country 并未重新赋值，所以还是之前定义的名字。如果将中间的代码改为({ firstname, lastname , country} = student);。那么, country 的值也会被重新赋值，为'Nigeria'

下面看下嵌套对象的结构：

const student = {
+  name: 'John Doe',
+  age: 16,
+  scores: {
+    maths: 74,
+    english: 63,
+  },
+}
+
+// We define 3 local variables: name, maths, science
+const {
+  name,
+  scores: { maths, science = 50 },
+} = student
+
+console.log(
+  `${name} scored ${maths} in Maths and ${science} in Elementary Science.`
+)
+
+// John Doe scored 74 in Maths and 50 in Elementary Science.
+
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使用嵌套对象解构时，要小心避免使用空的嵌套对象字面量。虽然它是有效的语法，但它实际上没有赋值。例如，上面的 science，如果改成 const { name, scores: {maths, science} } = student;。那么science的值将是undefined

点运算配合解构赋值：

const person = {
+  name: 'John Doe',
+  country: 'Canada',
+  city: 'shenzhen',
+  org: '123',
+  id: '1',
+}
+
+// Assign default value of 25 to age if undefined
+const { id = '', ...all } = person
+console.log(all) //{ name: 'John Doe',country: 'Canada',city:'shenzhen',org:'123',}
+
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从案例中我们可以可以通过点运算+对象解构赋值，可以去除对象的属性，并将剩余属性赋值给all。

  • 从对象中取多个属性，有了新的处理。
  • 从对象中无损删除属性，有了新的处理。

当然我们也可以删除多个属性：

const person = {
+  name: 'John Doe',
+  country: 'Canada',
+  city: 'shenzhen',
+  org: '123',
+  id: '1',
+}
+
+// Assign default value of 25 to age if undefined
+const { id = '', name = '', ...all } = person
+console.log(all) //{country: 'Canada',city:'shenzhen',org:'123'}
+
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# 3.数组解构

在数组解构中，您在赋值表达式的左侧使用数组文字。数组文字上的每个变量名都映射到解构数组上同一索引处的相应项目。

const rgb = [255, 200, 0]
+
+// Array Destructuring
+const [red, green, blue] = rgb
+
+console.log(`R: ${red}, G: ${green}, B: ${blue}`) // R: 255, G: 200, B: 0
+
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# 逗号可以跳过元素

let [, , x] = [1, 2, 3, 4]
+console.log(x) // 3
+
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# 数组点运算

let [x, ...y] = [1, 2, 3, 4]
+console.log(x, y) // 1 [2,3,4]
+
+let [x, y, ...z] = ['a'] // x='a'; y=undefined; z=[]
+
+let [x, ...[y, z]] = ['a', 'b', 'c']
+// x = 'a'; y = 'b'; z = 'c'
+
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# 循环

for (let [key, value] of map) {
+  console.log(key + ' is ' + value)
+}
+
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# 交换值

;[x, y] = [y, x]
+
1

# 数组删除元素

let [first, ...rest] = ['a', 'b', 'c']
+// first = 'a'; rest = ['b', 'c']
+
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# 克隆数组

const rainbow = ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
+
+// Cloning with array destructuring and spread operator
+const [...rainbowClone] = rainbow
+
+console.log(rainbow === rainbowClone) // false
+console.log(rainbowClone) // ['red', 'orange', 'yellow', 'green', 'blue', 'indigo', 'violet']
+
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# 4.字符串解构

字符串也可以解构赋值。这是因为此时，字符串被转换成了一个类似数组的对象

const [a, b, c, d, e] = 'hello'
+a // "h"
+b // "e"
+c // "l"
+d // "l"
+e // "o"
+
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属性解构赋值

let { length: len } = 'hello'
+len // 5
+
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Last Updated: 2024-3-28 5:23:17 ├F10: PM┤

+ ← + + 字符串 + + → +

+ + + diff --git a/ES6+/index.html b/ES6+/index.html new file mode 100644 index 0000000..32df788 --- /dev/null +++ b/ES6+/index.html @@ -0,0 +1,54 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 1.前言

JavaScript 是目前世界上最常用的编程语言之一，被用于不同的平台，从网络浏览器、移动设备、虚拟现实、网络服务器等。尽管与其他编程语言相比，该语言多年来没有太大变化，但一些变化值得注意。

一般来说，完整的 JavaScript 包括以下几个部分：

  • ECMAScript，描述了该语言的语法和基本对象；
  • 文档对象模型（DOM），描述处理网页内容的方法和接口；
  • 浏览器对象模型（BOM），描述与浏览器进行交互的方法和接口。

2015 年 6 月，ECMAScript 的第 6 个版本推出，这是我们称之为 ES2015 或 ES6 的 javascript 的最大转变。学习 ES6 很重要，因为一个原因是它使 JavaScript 更好、更易于编写，而且 ES6 还与当今的现代 Web 技术（如 React、Vue、Node. js 等）一起使用。

ES6（ECMAScript 6）是 JavaScript 的一种新的标准，也被称为 JavaScript 的下一代标准。它在 2015 年正式发布，为 JavaScript 带来了许多新的特性和语法糖，使得 JavaScript 更加强大和易于使用。以下是一些重要的 ES6 特性和语法：

  • 类
  • 模块化
  • 箭头函数
  • 函数参数默认值
  • 模板字符串
  • 解构赋值
  • 扩展操作符
  • 对象属性简写
  • Promise
  • let 与 const

以上是 ES6 的一些重要特性和语法，深入了解 ES6 可以使得 JavaScript 开发更加高效和愉快。 ES6 为 JavaScript 带来了许多新的特性和语法糖，使得 JavaScript 更加强大和易于使用。因此，对于 JavaScript 开发者来说，深入了解 ES6 是非常重要的。

后面 ES 还有几个版本的更新，也有很多特性以及语法的新增，但最大的更新还是 ES6 版本。

下面是 ES6+ 的一些笔记，如果对 ES6+ 的知识有所吸收，希望点下 star。

# 2.主要内容

# 3.开始旅程

让我们开始 ES6+的学习。如果文章有错误或者思考不周到之处，望不吝赐教！

写作是一件十分枯燥的事情，如果我写的这些文字对你有些许帮助的话，还请赏个 star 哈~~

Last Updated: 2024-3-25 5:50:41 ├F10: PM┤

+ ← + + 变量 + + → +

+ + + diff --git a/ES6+/number.html b/ES6+/number.html new file mode 100644 index 0000000..53c64a8 --- /dev/null +++ b/ES6+/number.html @@ -0,0 +1,249 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# BigInt

JavaScript 所有数字都保存成 64 位浮点数，这给数值的表示带来了两大限制。一是数值的精度只能到 53 个二进制位（相当于 16 个十进制位），大于这个范围的整数，JavaScript 是无法精确表示，这使得 JavaScript 不适合进行科学和金融方面的精确计算。二是大于或等于 2 的 1024 次方的数值，JavaScript 无法表示，会返回 Infinity。

ES2020 引入了一种新的数据类型 BigInt（大整数），来解决这个问题，这是 ECMAScript 的第八种数据类型。BigInt 只用来表示整数，没有位数的限制，任何位数的整数都可以精确表示。

# BigInt 类型

ES5: String, Number, Boolean, Null, Undefined
+ES6 Added: Symbol, 6 types
+ES10 added: BigInt, reaching 7 types

typeof 1n === 'bigint' // true
+typeof BigInt('1') === 'bigint' // true
+
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# BigInt & Number

过去，不支持大于 9007199254740992 的整数值。如果超过，该值将简单地锁定为 MAX_SAFE_INTEGER+1：

const limit = Number.MAX_SAFE_INTEGER
+console.log(limit) // 9007199254740991
+console.log(limit + 1) // 9007199254740992
+console.log(limit + 2) // 9007199254740992
+console.log(9007199254740993) // 9007199254740992
+
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然后我们看看 BigInt，BigInt 只用来表示整数，没有位数的限制，任何位数的整数都可以精确表示。

const larger = 9007199254740993n
+console.log(larger) // 9007199254740993n
+console.log(larger + 1n) // 9007199254740994n
+console.log(larger * 1000n) // 9007199254740993000n
+const largerNum = BigInt(9007199254740992)
+const largerNum2 = BigInt(9007199254740993)
+const largerStr = BigInt('9007199254740992')
+const largerStr2 = BigInt('9007199254740993')
+console.log(largerNum, largerNum2, largerStr, largerStr2) // 9007199254740992n 9007199254740992n 9007199254740992n 9007199254740993n
+console.log(
+  largerNum === largerStr,
+  largerNum == largerStr,
+  largerNum2 === largerStr
+) // true true true
+
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那么看看0n&0,1n&1对比如何

console.log(1n === 1, 1n == 1) // false true
+console.log(0n === 0, 0n == 0) // false true
+
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从上面式子不能看出 BigInt 值并不严格等于 Number 值。也就是不全等。

那两者的大小比较又会如何

1n < 2 // true
+2n > 1 // true
+2 > 2 // false
+2n > 2 // false
+2n >= 2 // true
+
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# BigInt 计算

我们再来看看 MDN 的示例，发现 + * - % ** 这些运算符都支持

const previousMaxSafe = BigInt(Number.MAX_SAFE_INTEGER)
+// 9007199254740991n
+
+const maxPlusOne = previousMaxSafe + 1n
+// 9007199254740992n
+
+const theFuture = previousMaxSafe + 2n
+// 9007199254740993n, this works now!
+
+const multi = previousMaxSafe * 2n
+// 18014398509481982n
+
+const subtr = multi - 10n
+// 18014398509481972n
+
+const mod = multi % 10n
+// 2n
+
+const bigN = 2n ** 54n
+// 18014398509481984n
+
+bigN * -1n
+// -18014398509481984n
+
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但是对于除法/和数字计算有些区别：

const expected = 4n / 2n
+// 2n
+
+const truncated = 5n / 2n
+// 2n, not 2.5n
+console.log(expected, truncated) // 2n 2n
+
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# BigInt 正负号

BigInt 可以使用负号（-），但是不能使用正号（+），因为会与 asm.js 冲突。

;-42n + // 正确
+  42n // 报错
+
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# BigInt 函数

JavaScript 原生提供 BigInt 函数，可以用它生成 BigInt 类型的数值。转换规则基本与 Number()一致，将其他类型的值转为 BigInt。

BigInt(123) // 123n
+BigInt('123') // 123n
+BigInt(false) // 0n
+BigInt(true) // 1n
+
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BigInt()函数必须有参数，而且参数必须可以正常转为数值，下面的用法都会报错。

new BigInt() // TypeError
+BigInt(undefined) //TypeError
+BigInt(null) // TypeError
+BigInt('123n') // SyntaxError
+BigInt('abc') // SyntaxError
+
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需要注意的是，BigInt 函数无法操作小数以及小数字符串

console.log(BigInt(123)) // 123n
+BigInt(123.3) // RangeError: The number 123.3 cannot be converted to a BigInt because it is not an integer
+BigInt(1.5) // RangeError
+BigInt('1.5') // SyntaxError
+
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BigInt 继承了 Object 对象的两个实例方法。

-- BigInt.prototype.toString()
+-- BigInt.prototype.valueOf()

它还继承了 Number 对象的一个实例方法。

-- BigInt.prototype.toLocaleString()

console.log(typeof Object(1n)) // "object"
+console.log(typeof Object(1n).valueOf()) // "bigint"
+
+console.log(1024n.toString()) //"1024"
+console.log(1024n.toString(2)) //"10000000000"
+console.log(1024n.toString(16)) //"400"
+
+const bigint = 123456789123456789n
+
+// German uses period for thousands
+console.log(bigint.toLocaleString('de-DE')) //"123.456.789.123.456.789"
+
+// Request a currency format
+console.log(
+  bigint.toLocaleString('de-DE', { style: 'currency', currency: 'EUR' })
+) //  "123.456.789.123.456.789,00 €"
+
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此外，还提供了三个静态方法。

-- BigInt.asUintN(width, BigInt)： 给定的 BigInt 转为 0 到 2width - 1 之间对应的值。
+-- BigInt.asIntN(width, BigInt)：给定的 BigInt 转为 -2width - 1 到 2width - 1 - 1 之间对应的值。
+-- BigInt.parseInt(string[, radix])：近似于 Number.parseInt()，将一个字符串转换成指定进制的 BigInt。

const max = 2n ** (64n - 1n) - 1n
+
+BigInt.asIntN(64, max)
+// 9223372036854775807n
+BigInt.asIntN(64, max + 1n)
+// -9223372036854775808n
+BigInt.asUintN(64, max + 1n)
+// 9223372036854775808n
+
+const max2 = 2n ** (64n - 1n) - 1n
+
+BigInt.asIntN(32, max2) // -1n
+BigInt.asUintN(32, max2) // 4294967295n
+
+// Number.parseInt() 与 BigInt.parseInt() 的对比
+Number.parseInt('9007199254740993', 10)
+// 9007199254740992
+BigInt.parseInt('9007199254740993', 10)
+// 9007199254740993n
+
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# Math 对象扩展

ES6 在 Math 对象上新增了 17 个与数学相关的方法。所有这些方法都是静态方法，只能在 Math 对象上调用。

# Math.trunc()

Math.trunc（）静态方法通过删除任何小数返回数字的整数部分。

console.log(Math.trunc(13.37)) //13
+
+console.log(Math.trunc(42.84)) //42
+
+console.log(Math.trunc(0.123)) //0
+
+console.log(Math.trunc(-0.123)) // -0
+
+console.log(Math.trunc('-1.123')) // -1
+
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一些特殊的，但是能转成数值的处理

Math.trunc(true) //1
+Math.trunc(false) // 0
+Math.trunc(null) // 0
+
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对于空值和无法截取整数的值，返回 NaN。

Math.trunc(NaN) // NaN
+Math.trunc('foo') // NaN
+Math.trunc() // NaN
+Math.trunc(undefined) // NaN
+
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处理Infinity/-Infinity，返回原值。

Math.trunc(-Infinity) // -Infinity
+Math.trunc(Infinity) // Infinity
+
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# Math.sign()

Math.sign 方法用来判断一个数到底是正数、负数、还是零。对于非数值，会先将其转换为数值。

它会返回五种值。

  • 参数为正数，返回+1；
  • 参数为负数，返回-1；
  • 参数为 0，返回 0；
  • 参数为-0，返回-0;
  • 其他值，返回 NaN。
Math.sign(3) // 1
+Math.sign(-3) // -1
+Math.sign('-3') // -1
+Math.sign(0) // 0
+Math.sign(-0) // -0
+Math.sign(NaN) // NaN
+Math.sign('foo') // NaN
+Math.sign() // NaN
+
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如果参数是非数值，会自动转为数值。对于那些无法转为数值的值，会返回 NaN。

Math.sign('') // 0
+Math.sign(true) // +1
+Math.sign(false) // 0
+Math.sign(null) // 0
+Math.sign('9') // +1
+Math.sign('foo') // NaN
+Math.sign() // NaN
+Math.sign(undefined) // NaN
+
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# Math.hypot()

Math.hypot 方法返回所有参数的平方和的平方根。 +如果参数不是数值，Math.hypot 方法会将其转为数值。只要有一个参数无法转为数值，就会返回 NaN。

Math.hypot(3, 4) // 5
+Math.hypot(3, 4, 5) // 7.0710678118654755
+Math.hypot() // 0
+Math.hypot(NaN) // NaN
+Math.hypot(3, 4, 'foo') // NaN
+Math.hypot(3, 4, '5') // 7.0710678118654755
+Math.hypot(-3) // 3
+
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# Math.cbrt()

Math.cbrt()方法用于计算一个数的立方根。

Math.cbrt(-1) // -1
+Math.cbrt(0) // 0
+Math.cbrt(1) // 1
+Math.cbrt(2) // 1.2599210498948732
+
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对于非数值，Math.cbrt()方法内部也是先使用 Number()方法将其转为数值。

Math.cbrt('8') // 2
+Math.cbrt('hello') // NaN
+
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# 32 位转换方法

  • Math.clz32()方法将参数转为 32 位无符号整数的形式，然后返回这个 32 位值里面有多少个前导 0。
  • Math.imul方法返回两个数以 32 位带符号整数形式相乘的结果，返回的也是一个 32 位的带符号整数。
  • Math.fround方法返回一个数的 32 位单精度浮点数形式。

# 对数方法

  • Math.expm1(x)返回 ex - 1，即 Math.exp(x) - 1。
  • Math.log1p(x)方法返回 1 + x 的自然对数，即 Math.log(1 + x)。如果 x 小于-1，返回 NaN。
  • Math.log10(x)返回以 10 为底的 x 的对数。如果 x 小于 0，则返回 NaN。
  • Math.log2(x)返回以 2 为底的 x 的对数。如果 x 小于 0，则返回 NaN。

# 双曲函数方法

  • Math.sinh(x) 返回 x 的双曲正弦（hyperbolic sine）
  • Math.cosh(x) 返回 x 的双曲余弦（hyperbolic cosine）
  • Math.tanh(x) 返回 x 的双曲正切（hyperbolic tangent）
  • Math.asinh(x) 返回 x 的反双曲正弦（inverse hyperbolic sine）
  • Math.acosh(x) 返回 x 的反双曲余弦（inverse hyperbolic cosine）
  • Math.atanh(x) 返回 x 的反双曲正切（inverse hyperbolic tangent）

# 二进制和八进制表示法

在 ES6 之前，对于整数的二进制表示，您最好的选择是将它们传递给基数为 2 的 parseInt。

parseInt('101', 2) //5
+
1

ES6 提供了二进制和八进制数值的新的写法，分别用前缀 0b（或 0B）和 0o（或 0O）表示。

0b111110111 === 503 // true
+0o767 === 503 // true
+0b111110111 === 503 // true
+0o767 === 503 // true
+
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ES6 延续了对以 10 为底的数字之外的数字文字表示方式的更改/变化。现在有了官方的八进制形式、修正的十六进制形式和全新的二进制形式。

let dec = 42,
+  oct = 0o52, // or `0O52`
+  hex = 0x2a, // or `0X2a`
+  bin = 0b101010 // or `0B101010`
+
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用 Number 处理这些字符串类型会被转化成字符串形式，

Number('42') // 42
+Number('0o52') // 42
+Number('0x2a') // 42
+Number('0b101010') // 42
+
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那么反过来处理会如何显示：

const a = 42
+
+a.toString() // "42" -- also `a.toString( 10 )`
+a.toString(8) // "52"
+a.toString(16) // "2a"
+a.toString(2) // "101010"
+
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# 数值分隔符

ES2021，允许 JavaScript 的数值使用下划线（_）作为分隔符。

let num = 12_345;
+console.log(num) // 12345
+num.toString() // '12345'
+
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数值分隔符有几个使用注意点。

  • 不能放在数值的最前面（leading）或最后面（trailing）。
  • 不能两个或两个以上的分隔符连在一起。
  • 小数点的前后不能有分隔符。
  • 科学计数法里面，表示指数的 e 或 E 前后不能有分隔符。
// 全部报错
+3_.141
+3._141
+1_e12
+1e_12
+123__456
+_1464301
+1464301_
+
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除了十进制，其他进制的数值也可以使用分隔符。

console.log(0b10_0) // 4
+console.log(0b10_0) // 4
+console.log(0o10_0) // 64
+console.log(0o10_0) // 64
+console.log(0x10_0) // 256
+
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Last Updated: 2024-4-1 11:24:19 ├F10: PM┤

+ ← + + 数值方法新增 + + → +

+ + + diff --git a/ES6+/numberMethod.html b/ES6+/numberMethod.html new file mode 100644 index 0000000..2a99265 --- /dev/null +++ b/ES6+/numberMethod.html @@ -0,0 +1,126 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 静态属性和 isSafeInteger

  • Number.EPSILON:ES6 在 Number 对象上面，新增一个极小的常量 Number.EPSILON。根据规格，它表示 1 与大于 1 的最小浮点数之间的差。
  • Number.MAX_SAFE_INTEGER:JS 数字值中明确表示的最大安全系数整数： 2^53-1
  • Number.MIN_SAFE_INTEGER:JS 数值中明确表示的最小安全系数整数：-（2^53-1）或（-2）^53

# Number.EPSILON

Number.EPSILON === Math.pow(2, -52)
+// true
+Number.EPSILON
+// 2.220446049250313e-16
+Number.EPSILON.toFixed(20)
+// "0.00000000000000022204"
+
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# Number.MAX_SAFE_INTEGER&Number.MIN_SAFE_INTEGER

Number.MAX_SAFE_INTEGER === Math.pow(2, 53) - 1
+// true
+Number.MAX_SAFE_INTEGER === 9007199254740991
+// true
+
+Number.MIN_SAFE_INTEGER === -Number.MAX_SAFE_INTEGER
+// true
+Number.MIN_SAFE_INTEGER === -9007199254740991
+// true
+
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# Number.isSafeInteger

Number.isSafeInteger('a') // false
+Number.isSafeInteger(null) // false
+Number.isSafeInteger(NaN) // false
+Number.isSafeInteger(Infinity) // false
+Number.isSafeInteger(-Infinity) // false
+
+Number.isSafeInteger(3) // true
+Number.isSafeInteger(1.2) // false
+Number.isSafeInteger(9007199254740990) // true
+Number.isSafeInteger(9007199254740992) // false
+
+Number.isSafeInteger(Number.MIN_SAFE_INTEGER - 1) // false
+Number.isSafeInteger(Number.MIN_SAFE_INTEGER) // true
+Number.isSafeInteger(Number.MAX_SAFE_INTEGER) // true
+Number.isSafeInteger(Number.MAX_SAFE_INTEGER + 1) // false
+
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# Number.isNaN

Number.isNaN()用来检查一个值是否为 NaN。如果参数类型不是 NaN，Number.isNaN 一律返回 false。

Number.isNaN(NaN) // true
+Number.isNaN(15) // false
+Number.isNaN('15') // false
+Number.isNaN(true) // false
+Number.isNaN(9 / NaN) // true
+Number.isNaN('true' / 0) // true
+Number.isNaN('true' / 'true') // true
+
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而 ES5 的 isNaN 方法，首先转换通过 Number 传递给它的值。下面的示例产生不一致的结果

isNaN('ponyfoo') // true
+
+isNaN(new Date()) // false
+
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isNaN()和 Number.isNaN()区别：

  • isNaN(value) 是判断 Number（value）转换后的值是否为 NaN;Number.isNaN(value)是判断 value。

我们可以通过 Number.isNaN()来判断 value 是否为数字类型

function isNumber(value) {
+  return typeof value === 'number' && !Number.isNaN(value)
+}
+
+isNumber(1) // true
+isNumber(Infinity) // true
+isNumber(NaN) // false
+isNumber('ponyfoo') // false
+isNumber(new Date()) // false
+
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# Number.isFinite

我们先看看isFinite的示例：

isFinite(NaN) // false
+isFinite(Infinity) // false
+isFinite(-Infinity) // false
+isFinite(null) // true, because Number(null) is 0
+isFinite('10') // true, because Number('10') is 10
+
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Number.isFinite()用来检查一个数值是否为有限的（finite），即不是 Infinity。

Number.isFinite(15) // true
+Number.isFinite(0.8) // true
+Number.isFinite(NaN) // false
+Number.isFinite(Infinity) // false
+Number.isFinite(-Infinity) // false
+Number.isFinite('foo') // false
+Number.isFinite('15') // false
+Number.isFinite(true) // false
+
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# Number.parseInt&Number.parseFloat()

ES6 将全局方法 parseInt()和 parseFloat()，移植到 Number 对象上面，行为完全保持不变。

// ES5的写法
+parseInt('12.34') // 12
+parseFloat('123.45#') // 123.45
+
+// ES6的写法
+Number.parseInt('12.34') // 12
+Number.parseFloat('123.45#') // 123.45
+
+Number.parseInt === parseInt // true
+Number.parseFloat === parseFloat // true
+
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Last Updated: 2024-4-1 11:24:19 ├F10: PM┤

+ ← + + 数组 + + → +

+ + + diff --git a/ES6+/string.html b/ES6+/string.html new file mode 100644 index 0000000..7eadaee --- /dev/null +++ b/ES6+/string.html @@ -0,0 +1,149 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 1.概览

模板字符串是允许嵌入表达式的字符串。您可以使用多行字符串、字符串插值和表达式插值功能。在 ES2015 规范的早期版本中，它们被称为“模板字符串”。

;`\`` === '`' // true
+;`\${1}` === '${1}' // true
+
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在下面这个例子中，字符串连接有两个令我印象深刻的恼人特性。用反斜杠转义撇号，并试图用双引号和单引号弄清楚字符串末尾发生了什么。模板文字减轻了这两个问题，我们留下了更清晰的代码行。

const p = {
+  name: 'Jackson',
+  nn: 'Jak',
+}
+// STRING CONCATENATION
+console.log("Hi, I'm " + p.name + '! Call me "' + p.nn + '".')
+// TEMPLATE LITERALS
+console.log(`Hi, I'm ${p.name}! Call me "${p.nn}".`)
+// "Hi, I'm Jackson! Call me 'Jak'."
+
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ES5

const a = 5
+const b = 10
+console.log('Fifteen is ' + (a + b) + ' and\nnot ' + (2 * a + b) + '.')
+// "Fifteen is 15 and
+// not 20."
+
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ES6

const a = 5
+const b = 10
+console.log(`Fifteen is ${a + b} and
+not ${2 * a + b}.`)
+// "Fifteen is 15 and
+// not 20."
+
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如您所见，在使用模板文字时，我们的代码更易于阅读。您可以轻松查看我们的字符串在说什么以及变量插入的位置。

对于换行符来说，我们可以看下面这个例子：

// STRING CONCATENATION
+console.log('Dear Mom,\n' + 'Hope you are well.\n' + '\tLove, your son')
+// TEMPLATE LITERALS
+console.log(`Dear Mom,
+Hope you are well.
+    Love, your son`)
+// Dear Mom,
+// Hope you are well.
+//     Love, your son
+
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使用模板字符串，插入任何新行字符、制表符、空格等都将成为字符串的一部分。这既是福也是祸，但就易读性而言绝对是一个优势。

不过，在 JavaScript 术语中，模板字符串是一种连接字符串的方法，同时允许嵌入表达式并提高易读性。

const person = 'Mike'
+const age = 28
+
+function myTag(strings, personExp, ageExp) {
+  const str0 = strings[0] // "That "
+  const str1 = strings[1] // " is a "
+  const str2 = strings[2] // "."
+
+  const ageStr = ageExp > 99 ? 'centenarian' : 'youngster'
+
+  // We can even return a string built using a template literal
+  return `${str0}${personExp}${str1}${ageStr}${str2}`
+}
+
+const output = myTag`That ${person} is a ${age}.`
+
+console.log(output)
+// That Mike is a youngster.
+
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# 模板字符串的特点：

  • 用反引号（`）标识。
  • 如果使用模板字符串表示多行字符串，所有的空格和缩进都会被保留在输出之中。
  • 模板字符串中嵌入变量，需要将变量名写在${}之中。一些运算也能嵌入其中。
  • 模板字符串甚至还能嵌套。

# 2.标签模板

模板字符串的功能，不仅仅是上面这些。它可以紧跟在一个函数名后面，该函数将被调用来处理这个模板字符串。这被称为“标签模板”功能（tagged template）。

function tagFunc(...args) {
+  return args
+}
+
+const setting = 'dark mode'
+const value = true
+console.log(
+  tagFunc`Setting ${setting} is ${value}!`, //[['Setting ', ' is ', '!'], 'dark mode', true]
+  [['Setting ', ' is ', '!'], 'dark mode', true]
+) //[['Setting ', ' is ', '!'], 'dark mode', true]
+
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第一个反引号之前的函数 tagFunc 称为标签函数。它的参数是：

  • 模板字符串（第一个参数）：一个数组，文本片段围绕插值${}。 +在示例中：['Setting '，' is ', '!']
  • 替换（剩余参数）：插值。 +在示例中：dark mode和true

字符串的静态（固定）部分（模板字符串）与动态部分（替换）保持分离。

# hello world

function tag(...e) {
+  console.log(e) // [['hello world']]
+}
+tag`hello world`
+
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从 hello world 中不难看出，标签模板中的字符串参数会被放在数组内。我们现在看下加入插值后会发生什么变化。

# 加入插值${}

function tag(...e) {
+  console.log(e) // [['hello world ', ""], '123']
+}
+const a = '123'
+tag`hello world ${a}`
+
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其实 tag 相当于传入tag(['hello world ', ""], '123')

function tag(...e) {
+  console.log(e) // [['hello world ', ""], '123']
+}
+const a = '123'
+tag`hello world ${a}`
+
+tag(['hello world ', ''], '123') // [['hello world ', ""], '123']
+
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# 拼合

let total = 30
+let msg = passthru`The total is ${total} (${total * 1.05} with tax)`
+
+function passthru(literals) {
+  let result = ''
+  let i = 0
+
+  while (i < literals.length) {
+    result += literals[i++]
+    if (i < arguments.length) {
+      result += arguments[i]
+    }
+  }
+
+  return result
+}
+
+console.log(msg) // "The total is 30 (31.5 with tax)"
+
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Last Updated: 2024-3-28 5:23:17 ├F10: PM┤

+ ← + + 字符串ES6+ + + → +

+ + + diff --git a/ES6+/stringES6+.html b/ES6+/stringES6+.html new file mode 100644 index 0000000..1371456 --- /dev/null +++ b/ES6+/stringES6+.html @@ -0,0 +1,112 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 概览

ES2017 引入了字符串补全长度的功能。如果某个字符串不够指定长度，会在头部或尾部补全。

ES2019 对字符串实例新增了 trimStart()和 trimEnd()这两个方法。

ES2020 增加了 String.prototype.matchAll()方法，可以一次性取出所有匹配。不过，它返回的是一个遍历器（Iterator），而不是数组。

ES2021 引入了 replaceAll()方法，可以一次性替换所有匹配。

# 1.padStart & padEnd

padStart()用于头部补全（左侧）
+padEnd()用于尾部补全(右侧)

'cat'.padStart(5) // '  cat'
+'cat'.padStart(5, 'a') // 'aacat'
+
+'cat'.padEnd(5) // 'cat  '
+'cat'.padEnd(5, 'a') // 'cataa'
+
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上面代码中，padStart()和 padEnd()一共接受两个参数，第一个参数是字符串补全生效的最大长度，第二个参数是用来补全的字符串。

如果原字符串的长度，等于或大于最大长度，则字符串补全不生效，返回原字符串。

'cat'.padStart(1, 'a') // 'cat'
+
+'cat'.padEnd(1, 'a') // 'cat'
+
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如果用来补全的字符串与原字符串，两者的长度之和超过了最大长度，则会截去超出位数的补全字符串。

'cat'.padStart(5, 'abc') // 'abcat'
+
+'cat'.padEnd(5, 'abc') // 'catab'
+
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如果省略第二个参数，默认使用空格补全长度。

'cat'.padStart(5) // '  cat'
+
+'cat'.padEnd(5) // 'cat  '
+
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# 2.trimStart & trimEnd

我们已经有了 trim（）函数。它删除字符串中的空格。现在新添加的是 trimStart（）和 trimEnd（）函数，它们帮助您选择字符串的哪一部分需要修剪。

const mySample = “ &nbsp;&nbsp;&nbsp;Hey medium “;
+console.log(mySample.trimStart());
+// “Hey medium&nbsp;&nbsp; “;
+console.log(mySample.trimEnd());
+// “ &nbsp;&nbsp;&nbsp;Hey There”;
+
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除了空格键，这两个方法对字符串头部（或尾部）的 tab 键、换行符等不可见的空白符号也有效。

浏览器还部署了额外的两个方法，trimLeft()是 trimStart()的别名，trimRight()是 trimEnd()的别名。

String.prototype.trimLeft.name === 'trimStart' // true
+String.prototype.trimRight.name === 'trimEnd' // true
+
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# 3.matchAll

给定一个字符串和一个正则表达式，matchAll（）返回所有匹配项的匹配对象的迭代器。

我们先看下 match 如何匹配的

let string = "Hello";
+let matches = string.match(/l/);
+console.log(matches); // "l"
+['l', index: 2, input: 'Hello', groups: undefined]
+
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带有正则表达式和 /g 标志的 String. match 确实返回多个匹配项：

let string = 'Hello'
+let matches = string.match(/l/g)
+console.log(matches) // "l"
+;['l', 'l']
+
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虽然输出了多个匹配项，明显结果不一样。

let iterator = 'hello'.matchAll(/[el]/g)
+for (const match of iterator) console.log(match)
+// ['e', index: 1, input: 'hello', groups: undefined] // Iteration 1
+// ['l', index: 2, input: 'hello', groups: undefined] // Iteration 2
+// ['l', index: 3, input: 'hello', groups: undefined] // Iteration 3
+
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const regexp = /t(e)(st(\d?))/g
+const str = 'test1test2'
+
+const array = [...str.matchAll(regexp)]
+
+console.log(array[0])
+// Expected output: Array ["test1", "e", "st1", "1"]
+
+console.log(array[1])
+// Expected output: Array ["test2", "e", "st2", "2"]
+
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# 4.replaceAll

replace eAll（）方法返回一个新字符串，其中模式的所有匹配项都被替换。该模式可以是字符串或 RegExp，替换可以是字符串或要为每个匹配项调用的函数。原始字符串保持不变。

let str = 'aabbcc'
+let newStr = str.replaceAll('b', '_')
+console.log(str, newStr) // 'aabbcc' 'aa__cc'
+
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上面代码不难看出：replaceAll 操作字符串后，原始字符串保持不变。

和 replace 替换空字符串对比

'xxx'.replaceAll('', '_') // "_x_x_x_"
+'xxx'.replace('', '_') // "_xxx"
+
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非全局正则表达式报错

// 不报错
+'aabbcc'.replace(/b/, '_')
+
+// 不报错
+'aabbcc'.replaceAll(/b/g, '_')
+
+// 报错
+'aabbcc'.replaceAll(/b/, '_')
+
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Last Updated: 2024-3-28 5:31:33 ├F10: PM┤

+ ← + + 数值类型 + + → +

+ + + diff --git a/ES6+/stringES6.html b/ES6+/stringES6.html new file mode 100644 index 0000000..e366e7c --- /dev/null +++ b/ES6+/stringES6.html @@ -0,0 +1,183 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# unicode 相关方法

JavaScript 字符串使用 UTF-16 代码单元表示。每个代码单元可用于表示[U+0000， U+FFFF]范围内的代码点——也称为“基本多语言平面”（BMP）。您可以使用“”语法表示 BMP 平面中的单个代码点。您还可以使用\x00…\xff 表示法表示[U+0000，U+0255]中的代码单元。

例如：表情字符串

'\ud83d\udc0e\ud83d\udc71\u2764'
+// '🐎👱❤'
+
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'\ud83d\udc0e\ud83d\udc71\u2764'.length // 5
+'🐎👱❤'.length //5
+
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虽然该字符串由 5 个代码单元组成，但我们知道长度实际上应该是三个-因为只有三个表情符号。

以 Object. key 为例，仍然有五个代码单元长。

console.log(Object.keys('🐎👱❤')) // ['0', '1', '2', '3', '4']
+
1

用 for 循环也得不到自己想要的

const text = '🐎👱❤'
+for (let i = 0; i < text.length; i++) {
+  console.log(text[i])
+  // <- '?'
+  // <- '?'
+  // <- '?'
+  // <- '?'
+  // <- '❤'
+}
+
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在 ES6 中，我们可以使用字符串迭代器来检查代码点。字符串可迭代对象生成的迭代器意识到代码单元循环的这种限制，因此它们会产生代码点。

for (let codePoint of '🐎👱❤') {
+  console.log(codePoint)
+  // <- '🐎'
+  // <- '👱'
+  // <- '❤'
+}
+
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;[...'🐎👱❤'].length // 3
+
1

# String.prototype.codePointAt

ES6 提供了 codePointAt()方法，能够正确处理 4 个字节储存的字符，返回一个字符的码点。

'\ud83d\udc0e\ud83d\udc71\u2764'.codePointAt(0)
+// 128014
+'\ud83d\udc0e\ud83d\udc71\u2764'.codePointAt(2)
+// 128113
+'\ud83d\udc0e\ud83d\udc71\u2764'.codePointAt(4)
+// 10084
+
+for (let codePoint of '\ud83d\udc0e\ud83d\udc71\u2764') {
+  console.log(codePoint.codePointAt(0))
+  // 128014
+  // 128113
+  // 10084
+}
+
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点运算+map

;[...'\ud83d\udc0e\ud83d\udc71\u2764'].map((cp) => cp.codePointAt(0))
+// [128014, 128113, 10084]
+
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然后，您可以使用新的 unicode 代码点转义语法\u{codePoint}将这些以 10 为底的整数的十六进制（base-16）表示并将它们呈现在字符串上。此语法允许您表示超出“基本多语言平面”（BMP）的 unicode 代码点，即通常使用语法表示的[U+0000， U+FFFF]范围之外的代码点。

for (let codePoint of '\ud83d\udc0e\ud83d\udc71\u2764') {
+  let a = codePoint.codePointAt(0).toString(16)
+  console.log(a)
+  // '1f40e'
+  // '1f471'
+  // '2764'
+}
+console.log('\u{1f40e}') // 🐎
+console.log('\u{1f471}') // 👱
+console.log('\u{2764}') // ❤
+
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9
10

# String.fromCodePoint

ES5 提供 String.fromCharCode()方法，用于从 Unicode 码点返回对应字符，但是这个方法不能识别码点大于 0xFFFF 的字符。 ES6 提供了 String.fromCodePoint()方法，可以识别大于 0xFFFF 的字符，弥补了 String.fromCharCode()方法的不足。

请注意，我如何将 0x 前缀与我们刚才从. codePointAt 获得的简洁的以 16 为底的代码点一起使用。

String.fromCodePoint(0x1f40e)
+// '🐎'
+String.fromCodePoint(0x1f471)
+// '👱'
+String.fromCodePoint(0x2764)
+// '❤'
+
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显然，你也可以使用他们的以 10 为底的对应物来达到同样的结果。

String.fromCodePoint(128014)
+// '🐎'
+String.fromCodePoint(128113)
+// '👱'
+String.fromCodePoint(10084)
+// '❤'
+
+String.fromCodePoint(128014, 128113, 10084) // '🐎👱❤'
+
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fromCodePoint + codePointAt

String.fromCodePoint(
+  ...[...'\ud83d\udc0e\ud83d\udc71\u2764'].map((cp) => cp.codePointAt(0))
+) // '🐎👱❤'
+
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# String.prototype.normalize

ES6 提供字符串实例的 normalize()方法，用来将字符的不同表示方法统一为同样的形式，这称为 Unicode 正规化。

'mañana' === 'mañana' // false
+
+'mañana'.length
+// 6
+'mañana'.length
+// 7
+
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normalize 操作这两个字符串后：

'mañana'.normalize() === 'mañana'.normalize()
+
+function compare(left, right) {
+  return left.normalize() === right.normalize()
+}
+
+console.log(compare('mañana', 'mañana')) // true
+compare('\x6d\x61\xf1\x61\x6e\x61', '\x6d\x61\x6e\u0303\x61\x6e\x61') // true
+
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我们可以在两个字符串上使用.normalize()来查看它们是否真的相等。

function compare(left, right) {
+  return left.normalize() === right.normalize()
+}
+compare('\x6d\x61\xf1\x61\x6e\x61', '\x6d\x61\x6e\u0303\x61\x6e\x61') // true
+compare('123', '123') // true
+
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# 原型方法

传统上，JavaScript 只有 indexOf 方法，可以用来确定一个字符串是否包含在另一个字符串中。ES6 又提供了三种新方法。

  • includes()：返回布尔值，表示是否找到了参数字符串。
  • startsWith()：返回布尔值，表示参数字符串是否在原字符串的头部。
  • endsWith()：返回布尔值，表示参数字符串是否在原字符串的尾部。

# 1.String.prototype.startsWith

一个非常常见的问题是“这个字符串是否以第一个字符串开头？”

在 ES5 中，我们一般使用 indexOf：

const foo = 'foo'
+console.log(foo.indexOf('fo')) // 0
+
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ES6 的话，我们可以 startsWidth 来判断是否是第一个字符

const foo = 'foo'
+console.log(foo.startsWith('fo')) // true
+
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不仅可以判断是否第一个字符，我们还可以判断该字符在其他位置出现是否正确。例如

const foo = 'foofoofoo'
+console.log(foo.startsWith('foo', 0)) // true
+console.log(foo.startsWith('foo', 3)) // true
+console.log(foo.startsWith('foo', 6)) // true
+console.log(foo.startsWith('foo', 5)) // false
+
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# 2.String.prototype.endsWith

endsWith 判断字符串是否以某段字符结尾。

'ponyfoo'.endsWith('foo') // true
+'ponyfoo'.endsWith('pony') // false
+
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就像. startsWith 一样，我们有一个位置索引来指示查找应该在哪里结束。它默认为字符串的长度。

'ponyfoo'.endsWith('foo', 7)
+// true
+'ponyfoo'.endsWith('pony', 0)
+// false
+'ponyfoo'.endsWith('pony', 4)
+// true
+
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# 3.String.prototype.includes

您可以使用.include 来确定一个字符串是否包含另一个字符串。

'foofoo'.includes('foo')
+// true
+'foofoo'.includes('sf')
+// false
+
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让我们来对比下.indexOf 和.includes 的使用。

'foofoo'.indexOf('foo') !== -1 // true
+'foofoo'.indexOf('zas') !== -1 // false
+
+'foofoo'.includes('oo', 1) // true
+'foofoo'.includes('oo', 4) // true
+'foofoo'.includes('oo', 2) // true
+'foofoo'.includes('oo', 5) // false
+
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# 3.String.prototype.repeat

repeat 方法返回一个新字符串，表示将原字符串重复 n 次。

'na'.repeat(0)
+// ''
+'na'.repeat(1)
+// 'na'
+'na'.repeat(2)
+// 'nana'
+'na'.repeat(5)
+// 'nanananana'
+
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提供的计数应该是一个正的有限数。

'na'.repeat(Infinity)
+// RangeError
+'na'.repeat(-1)
+// RangeError
+
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4

但是，如果参数是 0 到-1 之间的小数，则等同于 0，这是因为会先进行取整运算。0 到-1 之间的小数，取整以后等于-0，repeat 视同为 0。

'na'.repeat(-0.9) // ""
+
1

如果 repeat 的参数是字符串，则会先转换成数字。

'na'.repeat('na')
+// ''
+'na'.repeat('3')
+// 'nanana'
+
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4

参数 NaN 等同于 0。

'na'.repeat(NaN) // ""
+
1

小数会向下取整。

'na'.repeat(3.9) // 'nanana'
+
1
Last Updated: 2024-3-28 5:30:02 ├F10: PM┤

+ ← + + 模板字符串 + + → +

+ + + diff --git a/ES6+/variable.html b/ES6+/variable.html new file mode 100644 index 0000000..d49bdc9 --- /dev/null +++ b/ES6+/variable.html @@ -0,0 +1,239 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 1. 概览

ES6 新增了两个定义变量的关键字：let 与 const，它们几乎取代了 ES5 定义变量的方式：var。let是新的var,const简单的常量声明。

function f() {
+  {
+    let x
+    {
+      // okay, block scoped name
+      const x = 'sneaky'
+      // error, const
+      x = 'foo'
+    }
+    // error, already declared in block
+    let x = 'inner'
+  }
+}
+
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ES6 之前变量之前存在两个问题：

  • JS 没有块级作用域。在 JS 函数中的 var 声明，其作用域是函数体的全部。
  • 循环内变量过度共享

# 2. 块级作用域

let,const创建的变量都是块级作用域：它们只存在包围它们的最深代码块中。

作用域有哪些？

  • 块级作用域
  • 函数作用域
  • 全局作用域
function func() {
+  if (true) {
+    let tmp = 123
+    // const tmp = 123;
+  }
+  console.log(tmp) // ReferenceError: tmp is not defined
+}
+console.log(tmp) // ReferenceError: tmp is not defined
+
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相比之下，var声明的是函数域。

function func() {
+  if (true) {
+    var tmp = 123
+  }
+  console.log(tmp) // 123
+}
+func()
+console.log(tmp) // tmp is not defined
+
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8

下面的式子更难看出函数作用域与块级作用域的区别

function myFunc() {
+  var number1 = 15 // 函数作用域
+  if (true) {
+    let number1 = 20 // 块级作用域
+    console.log(number1) // 20
+  }
+  console.log(number1) // 15
+}
+myFunc()
+console.log(number1) // number1 is not defined
+
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而全局作用域，可以从 JavaScript 程序中的任何位置访问。

var number1 = 15
+console.log(window.number1, globalThis.number1, window === globalThis) // 15 15 true
+
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面试题：循环中定时器闭包

for (var i = 0; i < 5; i++) {
+  setTimeout(() => {
+    console.log(i) //5, 5, 5, 5, 5
+  }, 0)
+}
+console.log(i) //5 i跳出循环体污染外部函数
+
+//将var改成let之后
+for (let i = 0; i < 5; i++) {
+  setTimeout(() => {
+    console.log(i) // 0,1,2,3,4
+  }, 0)
+}
+console.log(i) //i is not defined i无法污染外部函数
+
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在 for 循环中使用 var 声明的循环变量，会跳出循环体污染当前的函数。

# 3. TDZ

let、const暂时性死区（temporal dead zone）

let,const声明的变量拥有暂时性死区：当进入它的作用域，它不能被访问（获取或设置）直到执行到达声明。
+简单描述：

if (true) {
+  //这块区域是TDZ
+  console.log(a) // Uncaught ReferenceError: Cannot access 'a' before initialization
+  let a = 1
+  // const a = 1
+}
+
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if (true) {
+  // enter new scope, TDZ starts
+  // Uninitialized binding for `tmp` is created
+
+  tmp = 'abc' // ReferenceError
+  console.log(tmp) // ReferenceError
+
+  let tmp // TDZ ends, `tmp` is initialized with `undefined`
+  console.log(tmp) // undefined
+
+  tmp = 123
+  console.log(tmp) // 123
+}
+
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下面示例将演示死区（dead zone）是真正短暂时间的（基于时间）和不受空间条件限制（基于位置）

if (true) {
+  // enter new scope, TDZ starts
+  const func = function () {
+    console.log(myVar) // OK!
+  }
+
+  // Here we are within the TDZ and
+  // accessing `myVar` would cause a `ReferenceError`
+
+  let myVar = 3 // TDZ ends
+  func() // called outside TDZ
+}
+
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上面的例子我们可以清楚的看到 let myVar 变量被定义了，但是不像 var 一样被提升。那它初始化了吗？没有，这就是 TDZ 在进入块级作用域就存在的原因。基本上，当 let myVar 将被定义时，它将进入 TDZ，并在您声明和初始化它时结束。所以在范围中，它将首先在 TDZ 中，但是一旦到达声明部分，TDZ 就会结束。

# 为什么会存在 TDZ?

  • 它帮助我们发现错误。
  • 在声明变量之前尝试访问它是错误的方式。

为避免 TDZ，请始终确保在任何范围的顶部定义 let 和 const。

我们可以通过分解来简单地理解这个术语。时间意味着暂时的东西，死亡意味着没有生命的状态，编程世界中与内存相关的区域。所以变量暂时不可用（或死亡）的时区在 TDZ 中。

或者简单地说，进入块级作用域与其变量创建声明之间的时间跨度称为时间死区。

# 4. 变量提升

var变量提升

JavaScript 中，我们通常说的作用域是函数作用域，使用 var 声明的变量，无论是在代码的哪个地方声明的，都会提升到当前作用域的最顶部，这种行为叫做变量提升（Hoisting）

下面代码，演示了函数的变量提升：

{
+  // Enter a new scope
+
+  console.log(foo()) // hello, due to hoisting
+  function foo() {
+    return 'hello'
+  }
+}
+
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也就是说，如果在函数内部声明的变量，都会被提升到函数开头，而在全局的声明，就会提升到全局作用域的顶部。

function test() {
+  console.log('1: ', a) //undefined
+  if (false) {
+    var a = 1
+  }
+  console.log('3: ', a) //undefined
+}
+
+test()
+
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实际执行时，上面的代码中的变量 a 会提升到函数顶部声明，即使 if 语句的条件是 false，也一样不影响 a 的提升。

function test() {
+  var a
+  //a声明没有赋值
+  console.log('1: ', a) //undefined
+  if (false) {
+    a = 1
+  }
+  //a声明没有赋值
+  console.log('3: ', a) //undefined
+}
+
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在嵌套函数的情况，变量只会提升到最近一个函数的顶部，而不会到外部函数。

//b提升到函数a顶部，但不会提升到函数test。
+function test() {
+  function a() {
+    if (false) {
+      var b = 2
+    }
+  }
+  console.log('b: ', b)
+}
+
+test() //b is not defined
+
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# 5. 不允许重复声明

let不允许重复声明

let不允许在相同作用域内，重复声明同一个变量。

// 报错
+function func() {
+  let a = 10
+  var a = 1
+}
+
+// 报错
+function func() {
+  let a = 10
+  let a = 1
+}
+
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因此在函数内部不能重新声明函数

function func(arg) {
+  let arg
+}
+func() // 报错 Identifier 'arg' has already been declared
+
+function func(arg) {
+  {
+    let arg
+  }
+}
+func() // 不报错
+
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# 6. const 命令

const 声明一个只读的常量。一旦声明，常量的值就不能改变。

一般使用场景：

const start = 'hi all'
+
+const getName = () => {
+  return 'jelly'
+}
+
+const conf = {
+  fav: 'Coding',
+}
+
+// 模板
+const msg = `${start}, my name is ${getName()}, ${conf.fav} is my favourite`
+
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你可能不知道的事：

// 1. 与引号混用
+const wantToSay = `I'm a "tbfed"`
+
+// 2. 支持多行文本
+const slogan = `
+I have a dream today!
+`
+
+// 比较适合写HTML
+const resultTpl = `
+  <section>
+    <div>...</div>
+  </section>
+`
+
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在 JavaScript 中，const 仅表示绑定（变量名和变量值之间的关联）是不可变的。值本身可能是可变的，如以下示例中的 obj。

const obj = { prop: 0 }
+obj.prop = obj.prop + 1
+console.log(obj.prop) // 1
+
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# 7. 全局变量和全局对象

在 Web 浏览器中，唯一直接位于该范围内的位置是脚本的顶层。全局范围的变量称为全局变量，可以随处访问。有两种全局变量：

  • 全局声明变量是普通变量。 +它们只能在脚本的顶层通过 const、let 和类声明创建。
  • 全局对象变量被存储在所谓全局对象的属性中。 +它们是在脚本的顶层通过 var 和函数声明创建的。 +可以通过全局变量 globalThis 访问全局对象，它可以用来创建、读取和删除全局对象变量。 +除此之外，全局对象变量像普通变量一样工作。
window === globalThis // true
+
+console.log(window.a) // 1
+var a = 1
+console.log(window.a) // 1
+
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# 8. 区别

var、let和const是 JavaScript 用来存储和声明变量的特殊关键字。它们每个都有唯一性（差异），将简要讨论。

  • 相同点：var,let,const声明的变量，是不能被delete的;
  • 区别：

var:

  • var 分别具有全局和函数作用域，也就是说，定义在函数外部的变量可以全局访问，定义在特定函数内部的变量只能在函数内部访问。
  • 其次，用户可以使用 var 重新声明变量，用户可以更新 var 变量。
  • 如果用户在声明之前使用 var 变量，它会使用未定义的值进行初始化，值为 undefined。

let:

  • let 变量的作用域仅为块作用域。它不能在特定功能块之外访问，let 关键字是 var 关键字的改进版本。
  • 用户不能重新声明使用 let 关键字定义的变量，但可以更新它。
  • 用户可以使用 let 关键字在不同的功能块中声明同名变量。
  • 无需初始化即可声明。

const:

  • const 变量的作用域是块作用域。
  • 它不能更新或重新声明到范围内
  • 没有初始化就不能声明

变量提升：var声明的变量存在变量提升，即变量可以在声明之前调用，值为 undefined；
let,const不存在变量提升，即它们声明的变量一定要在声明后使用，否则会报错。

暂时性死区：var不存在暂时性死区；let、const存在暂时性死区，只有等声明变量后，才可以获取和使用该变量。

重复声明：var允许重复声明；lat、const在同一作用域不允许重复声明。

修改声明的变量：var和let可以修改声明的变量；const声明一个只读常量，一旦声明，常量的值就不能改变。

# 9. 面试题

# 1.请问 var、let 和 const 有何区别？

作用域不同：

var 声明的变量的作用域是当前执行上下文，或者说对于声明在任何函数外的变量来说是全局的

let 、 const 声明的是块级作用域变量，只在它所在的代码块内有效

变量提升现象：

var 声明的变量会被提升到作用域顶部，并初始化为 undefined

let、 const 声明的变量会被提升到顶部，但在声明代码前不能使用（暂时性死区），会被初始化 undefined

变量/常量：

var 和 let 声明变量，const 声明只读常量

暂时性死区：

var 可以先使用，后声明，值为 undefined

let 、const 必须先声明，后使用；且 const 必须初始化赋值

在 let、const 声明之前就访问对应的变量与常量，会抛出 ReferenceError，产生原因：

由 let/const 声明的变量，当它们包含的词法环境(Lexical Environment)被实例化时会被创建，但只有在变量的词法绑定(LexicalBinding)已经被求值运算后，才能够被访问（摘自 ES2015 语言标准）

简单来说：用 let/const 声明的变量会先在作用域中被创建出来，但此时还未进行词法绑定，是不能被访问的，会抛出错误。从在作用域创建变量开始，到变量开始可被访问之间的一段时间，称之为 TDZ(暂时性死区)

# 2.const 声明的变量是绝对的不可变吗？

实际上，不是 const 变量的值不得改动，而是变量指向的那个内存地址不得改动。对于基础类型数据，值保存在变量指向的那个内存地址，等同于常量

对于复合类型数据（主要是对象和数组），变量指向的内存地址，保存的只是一个指针，const 只能保证这个指针是不变的，至于它指向的数据结构是否变化，是不可控的。

例如 const 定义一个对象，对象的属性的值是可变的。

Last Updated: 2024-3-28 5:23:17 ├F10: PM┤

+ ← + + 解构赋值 + + → +

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ES6 新增的数据结构 Set 和 Map）。")])]),s._v(" "),t("p",[s._v("JavaScript 中的“类数组对象”是一个具有长度属性的对象，特别是整数值为零或更高的对象。"),t("br"),s._v("\n在 ES6 之前我们需要这样转化成数组")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// array-like object类似数组的对象，含有length属性")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" arrLike "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("length")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token 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function"}},[s._v("slice")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("call")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arrLike"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arr"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ['foo', 'bar', empty] 虽然浏览器显示的empty,但实际是没有第三个值的，长度为3，第三个值打印出来会是undefined")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br")])]),t("p",[s._v("slice()可以用作复制数组")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" arrLike "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n 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"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("prototype"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("slice")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("call")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arrLike"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" arr2 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" arr"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("slice")]),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("而用 ES6 的 Array.from 来操作转换数组和复制的操作，更加优雅简洁。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" arrLike "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("length")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'foo'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'bar'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token 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punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("push")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arr"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" arr2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ['foo', 'bar', undefined] ['foo', 'bar', undefined, 2]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[t("code",[s._v("Array.from")]),s._v(" 可以传三个参数，第一个参数可以是类数组（）或可遍历对象（）。第二个参数是函数。如果 map()函数里面用到了 this 关键字，还可以传入 Array.from()的第三个参数，用来绑定 this。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("Array"),t("span",{pre:!0,attrs:{class:"token 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thisArg"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br")])]),t("p",[s._v("第二个参数是函数，类似于操作 map 遍历操作内部数据。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("from")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("length")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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c"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// [empty × 4] [undefined, undefined, undefined, undefined] [undefined, undefined, undefined, undefined]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br")])]),t("p",[s._v("由于空位的处理规则非常不统一，所以建议避免出现空位。尽量让空位内有值 undefined。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("from")]),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("h2",{attrs:{id:"copywithin"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#copywithin"}},[s._v("#")]),s._v(" copyWithin")]),s._v(" "),t("blockquote",[t("p",[s._v("数组实例的 copyWithin()方法，在当前数组内部，将指定位置的成员复制到其他位置（会覆盖原有成员），然后返回当前数组。也就是说，使用这个方法，会修改当前数组。")])]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("prototype"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("copyWithin")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("target"),t("span",{pre:!0,attrs:{class:"token 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0]")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("fill")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token 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function"}},[s._v("fill")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// [{}, {}, {}]")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("fill")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// [function foo () {}, function foo () {}, function foo () {}]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("p",[s._v("语法：")]),s._v(" "),t("div",{staticClass:"language- line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-text"}},[t("code",[s._v("fill(value)\nfill(value, start)\nfill(value, start, end)\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br")])]),t("p",[s._v("注意，如果填充的类型为对象，那么被赋值的是同一个内存地址的对象，而不是深拷贝对象。")]),s._v(" "),t("div",{staticClass:"language-javascript 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"),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Mike'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\narr1"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("name "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Ben'")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arr1"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// [{name: "Ben"}, {name: "Ben"}, {name: "Ben"}]')]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" arr2 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("fill")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\narr2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("push")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("arr2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// [[5], [5], [5]]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br")])]),t("h2",{attrs:{id:"find"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#find"}},[s._v("#")]),s._v(" find")]),s._v(" "),t("blockquote",[t("p",[s._v("find() 方法返回数组中满足提供的测试函数的第一个元素的值。否则返回 undefined。")]),s._v(" "),t("p",[s._v("如果需要在数组中找到对应元素的索引，请使用 findIndex()。"),t("br"),s._v("\n如果需要查找某个值的索引，请使用 Array.prototype.indexOf()。（它类似于 findIndex()，但只是检查每个元素是否与值相等，而不是使用测试函数。）"),t("br"),s._v("\n如果需要查找数组中是否存在某个值，请使用 Array.prototype.includes()。同样，它检查每个元素是否与值相等，而不是使用测试函数。"),t("br"),s._v("\n如果需要查找是否有元素满足所提供的测试函数，请使用 Array.prototype.some()。")])]),s._v(" "),t("p",[s._v("语法：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token function"}},[s._v("find")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("callbackFn"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("find")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("callbackFn"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" thisArg"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("示例：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" arr1 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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string"}},[s._v("'Nigeria'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Object Destructuring")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" firstname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" lastname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" country "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" student\n\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("firstname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" lastname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" country"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Glad Chinda Nigeria")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("利用解构给变量重新赋值")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Initialize local variables")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" country "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Canada'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" firstname "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'John'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" lastname "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Doe'")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" student "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("firstname")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Glad'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("lastname")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Chinda'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("country")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Nigeria'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Reassign firstname and lastname using destructuring")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Enclose in a pair of parentheses, since this is an assignment expression")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" firstname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" lastname "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" student"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// country remains unchanged (Canada)")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("firstname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" lastname"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" country"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Glad Chinda Canada")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br"),t("span",{staticClass:"line-number"},[s._v("14")]),t("br"),t("span",{staticClass:"line-number"},[s._v("15")]),t("br"),t("span",{staticClass:"line-number"},[s._v("16")]),t("br"),t("span",{staticClass:"line-number"},[s._v("17")]),t("br")])]),t("p",[s._v("上面代码，用解构赋值的方式重新给 firstname, lastname 变量赋值，所以后面打印的时候，firstname, lastname 显示是 student 内部的 firstname, lastname。country 并未重新赋值，所以还是之前定义的名字。如果将中间的代码改为"),t("code",[s._v("({ firstname, lastname , country} = student);")]),s._v("。那么, country 的值也会被重新赋值，为'Nigeria'")]),s._v(" "),t("p",[s._v("下面看下嵌套对象的结构：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" student "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("name")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'John Doe'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("age")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("16")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("scores")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("maths")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("74")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("english")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("63")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// We define 3 local variables: name, maths, science")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n name"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("scores")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" maths"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" science "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("50")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" student\n\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token template-string"}},[t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")]),t("span",{pre:!0,attrs:{class:"token interpolation"}},[t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("${")]),s._v("name"),t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("}")])]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v(" scored ")]),t("span",{pre:!0,attrs:{class:"token interpolation"}},[t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("${")]),s._v("maths"),t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("}")])]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v(" in Maths and ")]),t("span",{pre:!0,attrs:{class:"token interpolation"}},[t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("${")]),s._v("science"),t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("}")])]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v(" in Elementary Science.")]),t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")])]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// John Doe scored 74 in Maths and 50 in Elementary Science.")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br"),t("span",{staticClass:"line-number"},[s._v("14")]),t("br"),t("span",{staticClass:"line-number"},[s._v("15")]),t("br"),t("span",{staticClass:"line-number"},[s._v("16")]),t("br"),t("span",{staticClass:"line-number"},[s._v("17")]),t("br"),t("span",{staticClass:"line-number"},[s._v("18")]),t("br"),t("span",{staticClass:"line-number"},[s._v("19")]),t("br"),t("span",{staticClass:"line-number"},[s._v("20")]),t("br")])]),t("p",[s._v("使用嵌套对象解构时，要小心避免使用空的嵌套对象字面量。虽然它是有效的语法，但它实际上没有赋值。例如，上面的 science，如果改成 "),t("code",[s._v("const { name, scores: {maths, science} } = student;")]),s._v("。那么"),t("code",[s._v("science")]),s._v("的值将是"),t("code",[s._v("undefined")])]),s._v(" "),t("p",[s._v("点运算配合解构赋值：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" person "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("name")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'John Doe'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("country")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Canada'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("city")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'shenzhen'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("org")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'123'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("id")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'1'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Assign default value of 25 to age if undefined")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" id "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("''")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),s._v("all "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" person\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("all"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//{ name: 'John Doe',country: 'Canada',city:'shenzhen',org:'123',}")]),s._v("\n")])]),s._v(" 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函数无法操作小数以及小数字符串")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("BigInt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("123")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 123n")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("BigInt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token 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10 为底的 x 的对数。如果 x 小于 0，则返回 NaN。")]),s._v(" "),t("li",[t("code",[s._v("Math.log2(x)")]),s._v("返回以 2 为底的 x 的对数。如果 x 小于 0，则返回 NaN。")])]),s._v(" "),t("h2",{attrs:{id:"双曲函数方法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#双曲函数方法"}},[s._v("#")]),s._v(" 双曲函数方法")]),s._v(" "),t("ul",[t("li",[t("code",[s._v("Math.sinh(x)")]),s._v(" 返回 x 的双曲正弦（hyperbolic sine）")]),s._v(" "),t("li",[t("code",[s._v("Math.cosh(x)")]),s._v(" 返回 x 的双曲余弦（hyperbolic cosine）")]),s._v(" "),t("li",[t("code",[s._v("Math.tanh(x)")]),s._v(" 返回 x 的双曲正切（hyperbolic tangent）")]),s._v(" "),t("li",[t("code",[s._v("Math.asinh(x)")]),s._v(" 返回 x 的反双曲正弦（inverse hyperbolic sine）")]),s._v(" "),t("li",[t("code",[s._v("Math.acosh(x)")]),s._v(" 返回 x 的反双曲余弦（inverse hyperbolic cosine）")]),s._v(" "),t("li",[t("code",[s._v("Math.atanh(x)")]),s._v(" 返回 x 的反双曲正切（inverse hyperbolic tangent）")])]),s._v(" "),t("h2",{attrs:{id:"二进制和八进制表示法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#二进制和八进制表示法"}},[s._v("#")]),s._v(" 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br")])]),t("p",[s._v("用 Number 处理这些字符串类型会被转化成字符串形式，")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token function"}},[s._v("Number")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'42'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 42")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("Number")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token 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"),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br")])]),s("p",[t._v("如您所见，在使用模板文字时，我们的代码更易于阅读。您可以轻松查看我们的字符串在说什么以及变量插入的位置。")]),t._v(" "),s("p",[t._v("对于换行符来说，我们可以看下面这个例子：")]),t._v(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// STRING CONCATENATION")]),t._v("\nconsole"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token 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using a template literal")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token template-string"}},[s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("str0"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("personExp"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("str1"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("ageStr"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("str2"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")])]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" output "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" myTag"),s("span",{pre:!0,attrs:{class:"token template-string"}},[s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("That ")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("person"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v(" is a ")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("age"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")])]),t._v("\n\nconsole"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("output"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// That Mike is a youngster.")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br"),s("span",{staticClass:"line-number"},[t._v("9")]),s("br"),s("span",{staticClass:"line-number"},[t._v("10")]),s("br"),s("span",{staticClass:"line-number"},[t._v("11")]),s("br"),s("span",{staticClass:"line-number"},[t._v("12")]),s("br"),s("span",{staticClass:"line-number"},[t._v("13")]),s("br"),s("span",{staticClass:"line-number"},[t._v("14")]),s("br"),s("span",{staticClass:"line-number"},[t._v("15")]),s("br"),s("span",{staticClass:"line-number"},[t._v("16")]),s("br"),s("span",{staticClass:"line-number"},[t._v("17")]),s("br"),s("span",{staticClass:"line-number"},[t._v("18")]),s("br")])]),s("h4",{attrs:{id:"模板字符串的特点"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#模板字符串的特点"}},[t._v("#")]),t._v(" 模板字符串的特点：")]),t._v(" "),s("ul",[s("li",[t._v("用反引号（`）标识。")]),t._v(" "),s("li",[t._v("如果使用模板字符串表示多行字符串，所有的空格和缩进都会被保留在输出之中。")]),t._v(" "),s("li",[t._v("模板字符串中嵌入变量，需要将变量名写在${}之中。一些运算也能嵌入其中。")]),t._v(" "),s("li",[t._v("模板字符串甚至还能嵌套。")])]),t._v(" "),s("h2",{attrs:{id:"_2-标签模板"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#_2-标签模板"}},[t._v("#")]),t._v(" 2.标签模板")]),t._v(" "),s("blockquote",[s("p",[t._v("模板字符串的功能，不仅仅是上面这些。它可以紧跟在一个函数名后面，该函数将被调用来处理这个模板字符串。这被称为“标签模板”功能（tagged template）。")])]),t._v(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("tagFunc")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("...")]),t._v("args")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" args\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("}")]),t._v("\n\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" setting "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'dark mode'")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" value "),s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token boolean"}},[t._v("true")]),t._v("\nconsole"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("\n tagFunc"),s("span",{pre:!0,attrs:{class:"token template-string"}},[s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("Setting ")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("setting"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v(" is ")]),s("span",{pre:!0,attrs:{class:"token interpolation"}},[s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("${")]),t._v("value"),s("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("!")]),s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")])]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//[['Setting ', ' is ', '!'], 'dark mode', true]")]),t._v("\n "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'Setting '")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("' is '")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'!'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token string"}},[t._v("'dark mode'")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(",")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token boolean"}},[t._v("true")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),t._v("\n"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token comment"}},[t._v("//[['Setting ', ' is ', '!'], 'dark mode', true]")]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br"),s("span",{staticClass:"line-number"},[t._v("6")]),s("br"),s("span",{staticClass:"line-number"},[t._v("7")]),s("br"),s("span",{staticClass:"line-number"},[t._v("8")]),s("br"),s("span",{staticClass:"line-number"},[t._v("9")]),s("br"),s("span",{staticClass:"line-number"},[t._v("10")]),s("br")])]),s("p",[t._v("第一个反引号之前的函数 tagFunc 称为标签函数。它的参数是：")]),t._v(" "),s("ul",[s("li",[t._v("模板字符串（第一个参数）：一个数组，文本片段围绕插值${}。\n在示例中：['Setting '，' is ', '!']")]),t._v(" "),s("li",[t._v("替换（剩余参数）：插值。\n在示例中："),s("code",[t._v("dark mode")]),t._v("和"),s("code",[t._v("true")])])]),t._v(" "),s("p",[t._v("字符串的静态（固定）部分（模板字符串）与动态部分（替换）保持分离。")]),t._v(" "),s("h4",{attrs:{id:"hello-world"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#hello-world"}},[t._v("#")]),t._v(" hello world")]),t._v(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("tag")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("...")]),t._v("e")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n console"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token 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"),s("h4",{attrs:{id:"加入插值"}},[s("a",{staticClass:"header-anchor",attrs:{href:"#加入插值"}},[t._v("#")]),t._v(" 加入插值"),s("code",[t._v("${}")])]),t._v(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("tag")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[s("span",{pre:!0,attrs:{class:"token operator"}},[t._v("...")]),t._v("e")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n console"),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("log")]),s("span",{pre:!0,attrs:{class:"token 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punctuation"}},[t._v("}")])]),s("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[t._v("`")])]),t._v("\n")])]),t._v(" "),s("div",{staticClass:"line-numbers-wrapper"},[s("span",{staticClass:"line-number"},[t._v("1")]),s("br"),s("span",{staticClass:"line-number"},[t._v("2")]),s("br"),s("span",{staticClass:"line-number"},[t._v("3")]),s("br"),s("span",{staticClass:"line-number"},[t._v("4")]),s("br"),s("span",{staticClass:"line-number"},[t._v("5")]),s("br")])]),s("p",[t._v("其实 tag 相当于传入"),s("code",[t._v("tag(['hello world ', \"\"], '123')")])]),t._v(" "),s("div",{staticClass:"language-javascript line-numbers-mode"},[s("pre",{pre:!0,attrs:{class:"language-javascript"}},[s("code",[s("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("function")]),t._v(" "),s("span",{pre:!0,attrs:{class:"token function"}},[t._v("tag")]),s("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),s("span",{pre:!0,attrs:{class:"token parameter"}},[s("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'cat'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("padStart")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ' cat'")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'cat'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("padEnd")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),t("span",{pre:!0,attrs:{class:"token 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operator"}},[s._v("&")]),s._v("nbsp"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("&")]),s._v("nbsp"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("&")]),s._v("nbsp"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("Hey medium “"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("mySample"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("trimStart")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// “Hey medium   “;")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("mySample"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("trimEnd")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// “    Hey There”;")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br")])]),t("p",[s._v("除了空格键，这两个方法对字符串头部（或尾部）的 tab 键、换行符等不可见的空白符号也有效。")]),s._v(" "),t("p",[s._v("浏览器还部署了额外的两个方法，trimLeft()是 trimStart()的别名，trimRight()是 trimEnd()的别名。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("prototype"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("trimLeft"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("name "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("===")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'trimStart'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// true")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("String")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("prototype"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("trimRight"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("name "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("===")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'trimEnd'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// true")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("h2",{attrs:{id:"_3-matchall"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_3-matchall"}},[s._v("#")]),s._v(" 3.matchAll")]),s._v(" "),t("blockquote",[t("p",[s._v("给定一个字符串和一个正则表达式，matchAll（）返回所有匹配项的匹配对象的迭代器。")])]),s._v(" "),t("p",[s._v("我们先看下 match 如何匹配的")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" string "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v('"Hello"')]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" matches "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" string"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("match")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token regex"}},[t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-source language-regex"}},[s._v("l")]),t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")])]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("matches"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// "l"')]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'l'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("index")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("input")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Hello'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("groups")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("undefined")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br")])]),t("p",[s._v("带有正则表达式和 /g 标志的 String. match 确实返回多个匹配项：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" string "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Hello'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" matches "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" string"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("match")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token regex"}},[t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-source language-regex"}},[s._v("l")]),t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-flags"}},[s._v("g")])]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("matches"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// "l"')]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'l'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'l'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br")])]),t("p",[s._v("虽然输出了多个匹配项，明显结果不一样。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" iterator "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'hello'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("matchAll")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token regex"}},[t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-source language-regex"}},[s._v("[el]")]),t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-flags"}},[s._v("g")])]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" match "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("of")]),s._v(" iterator"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("match"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ['e', index: 1, input: 'hello', groups: undefined] // Iteration 1")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ['l', index: 2, input: 'hello', groups: undefined] // Iteration 2")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ['l', index: 3, input: 'hello', groups: undefined] // Iteration 3")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br")])]),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" regexp "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token regex"}},[t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-source language-regex"}},[s._v("t(e)(st(\\d?))")]),t("span",{pre:!0,attrs:{class:"token regex-delimiter"}},[s._v("/")]),t("span",{pre:!0,attrs:{class:"token regex-flags"}},[s._v("g")])]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" str "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'test1test2'")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" array "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),s._v("str"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("matchAll")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("regexp"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// Expected output: Array ["test1", "e", "st1", "1"]')]),s._v("\n\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token 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comment"}},[s._v("// 5")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'🐎👱❤'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("length "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//5")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("虽然该字符串由 5 个代码单元组成，但我们知道长度实际上应该是三个-因为只有三个表情符号。")]),s._v(" "),t("p",[s._v("以 Object. key 为例，仍然有五个代码单元长。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("Object"),t("span",{pre:!0,attrs:{class:"token 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string"}},[s._v("'🐎👱❤'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" i "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" i "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),s._v(" text"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("length"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" i"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("++")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("text"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("i"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '?'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '?'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '?'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '?'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '❤'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br")])]),t("p",[s._v("在 ES6 中，我们可以使用字符串迭代器来检查代码点。字符串可迭代对象生成的迭代器意识到代码单元循环的这种限制，因此它们会产生代码点。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" codePoint "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("of")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'🐎👱❤'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("codePoint"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '🐎'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '👱'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// <- '❤'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'🐎👱❤'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("length "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 3")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br")])]),t("h3",{attrs:{id:"string-prototype-codepointat"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#string-prototype-codepointat"}},[s._v("#")]),s._v(" String.prototype.codePointAt")]),s._v(" "),t("blockquote",[t("p",[s._v("ES6 提供了 codePointAt()方法，能够正确处理 4 个字节储存的字符，返回一个字符的码点。")])]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\ud83d\\udc0e\\ud83d\\udc71\\u2764'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("codePointAt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 128014")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\ud83d\\udc0e\\ud83d\\udc71\\u2764'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("codePointAt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 128113")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\ud83d\\udc0e\\ud83d\\udc71\\u2764'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("codePointAt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("4")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 10084")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" codePoint "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("of")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\ud83d\\udc0e\\ud83d\\udc71\\u2764'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("codePoint"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("codePointAt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 128014")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 128113")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 10084")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br")])]),t("p",[s._v("点运算+map")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\ud83d\\udc0e\\ud83d\\udc71\\u2764'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("map")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("cp")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" cp"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("codePointAt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// [128014, 128113, 10084]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("然后，您可以使用新的 unicode 代码点转义语法\\u{codePoint}将这些以 10 为底的整数的十六进制（base-16）表示并将它们呈现在字符串上。此语法允许您表示超出“基本多语言平面”（BMP）的 unicode 代码点，即通常使用语法表示的[U+0000， U+FFFF]范围之外的代码点。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" codePoint "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("of")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\ud83d\\udc0e\\ud83d\\udc71\\u2764'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" a "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" codePoint"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("codePointAt")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("toString")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("16")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// '1f40e'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// '1f471'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// '2764'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\u{1f40e}'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 🐎")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\u{1f471}'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 👱")]),s._v("\nconsole"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'\\u{2764}'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ❤")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("h3",{attrs:{id:"string-fromcodepoint"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#string-fromcodepoint"}},[s._v("#")]),s._v(" String.fromCodePoint")]),s._v(" "),t("blockquote",[t("p",[s._v("ES5 提供 String.fromCharCode()方法，用于从 Unicode 码点返回对应字符，但是这个方法不能识别码点大于 0xFFFF 的字符。 ES6 提供了 String.fromCodePoint()方法，可以识别大于 0xFFFF 的字符，弥补了 String.fromCharCode()方法的不足。")])]),s._v(" "),t("p",[s._v("请注意，我如何将 0x 前缀与我们刚才从. codePointAt 获得的简洁的以 16 为底的代码点一起使用。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("String"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("fromCodePoint")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0x1f40e")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// '🐎'")]),s._v("\nString"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("fromCodePoint")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br")])]),t("p",[s._v("下面示例将演示死区（dead zone）是真正短暂时间的（基于时间）和不受空间条件限制（基于位置）")]),s._v(" "),t("div",{staticClass:"language-javascript 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"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("myVar"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// OK!")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Here we are within the TDZ and")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// accessing `myVar` would cause a `ReferenceError`")]),s._v("\n\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" myVar "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// TDZ ends")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("func")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// called outside TDZ")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br")])]),t("p",[s._v("上面的例子我们可以清楚的看到 "),t("code",[s._v("let myVar")]),s._v(" 变量被定义了，但是不像 "),t("code",[s._v("var")]),s._v(" 一样被提升。那它初始化了吗？没有，这就是 TDZ 在进入块级作用域就存在的原因。基本上，当 "),t("code",[s._v("let myVar")]),s._v(" 将被定义时，它将进入 TDZ，并在您声明和初始化它时结束。所以在范围中，它将首先在 TDZ 中，但是一旦到达声明部分，TDZ 就会结束。")]),s._v(" "),t("h4",{attrs:{id:"为什么会存在-tdz"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#为什么会存在-tdz"}},[s._v("#")]),s._v(" 为什么会存在 TDZ?")]),s._v(" "),t("ul",[t("li",[s._v("它帮助我们发现错误。")]),s._v(" "),t("li",[s._v("在声明变量之前尝试访问它是错误的方式。")])]),s._v(" "),t("blockquote",[t("p",[s._v("为避免 TDZ，请始终确保在任何范围的顶部定义 "),t("code",[s._v("let")]),s._v(" 和 "),t("code",[s._v("const")]),s._v("。")])]),s._v(" "),t("p",[s._v("我们可以通过分解来简单地理解这个术语。时间意味着暂时的东西，死亡意味着没有生命的状态，编程世界中与内存相关的区域。所以变量暂时不可用（或死亡）的时区在 TDZ 中。")]),s._v(" "),t("p",[s._v("或者简单地说，进入块级作用域与其变量创建声明之间的时间跨度称为时间死区。")]),s._v(" "),t("h2",{attrs:{id:"_4-变量提升"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_4-变量提升"}},[s._v("#")]),s._v(" 4. 变量提升")]),s._v(" "),t("blockquote",[t("p",[t("code",[s._v("var")]),s._v("变量提升")])]),s._v(" "),t("p",[s._v("JavaScript 中，我们通常说的作用域是函数作用域，使用 var 声明的变量，无论是在代码的哪个地方声明的，都会提升到当前作用域的最顶部，这种行为叫做"),t("strong",[s._v("变量提升（Hoisting）")])]),s._v(" "),t("p",[s._v("下面代码，演示了函数的变量提升：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Enter a new scope")]),s._v("\n\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// hello, due to hoisting")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'hello'")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br")])]),t("p",[s._v("也就是说，如果在函数内部声明的变量，都会被提升到函数开头，而在全局的声明，就会提升到全局作用域的顶部。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("test")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'1: '")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//undefined")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("if")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("false")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" a "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'3: '")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//undefined")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("test")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br")])]),t("p",[s._v("实际执行时，上面的代码中的变量 a 会提升到函数顶部声明，即使 if 语句的条件是 false，也一样不影响 a 的提升。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("test")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" a\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//a声明没有赋值")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'1: '")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//undefined")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("if")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("false")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n a "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//a声明没有赋值")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'3: '")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//undefined")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("在嵌套函数的情况，变量只会提升到最近一个函数的顶部，而不会到外部函数。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//b提升到函数a顶部，但不会提升到函数test。")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("test")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("a")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("if")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("false")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" b "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n console"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'b: '")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" b"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("test")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("//b is not defined")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br")])]),t("h2",{attrs:{id:"_5-不允许重复声明"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_5-不允许重复声明"}},[s._v("#")]),s._v(" 5. 不允许重复声明")]),s._v(" "),t("blockquote",[t("p",[t("code",[s._v("let")]),s._v("不允许重复声明")])]),s._v(" "),t("p",[t("code",[s._v("let")]),s._v("不允许在相同作用域内，重复声明同一个变量。")]),s._v(" 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comment"}},[s._v("// 不报错")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br")])]),t("h2",{attrs:{id:"_6-const-命令"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#_6-const-命令"}},[s._v("#")]),s._v(" 6. const 命令")]),s._v(" "),t("blockquote",[t("p",[s._v("const 声明一个只读的常量。一旦声明，常量的值就不能改变。")])]),s._v(" "),t("p",[s._v("一般使用场景：")]),s._v(" 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"),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'jelly'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" conf "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("fav")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Coding'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 模板")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" msg "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token template-string"}},[t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")]),t("span",{pre:!0,attrs:{class:"token interpolation"}},[t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("${")]),s._v("start"),t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("}")])]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v(", my name is ")]),t("span",{pre:!0,attrs:{class:"token interpolation"}},[t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("${")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getName")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("}")])]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v(", ")]),t("span",{pre:!0,attrs:{class:"token interpolation"}},[t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("${")]),s._v("conf"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("fav"),t("span",{pre:!0,attrs:{class:"token interpolation-punctuation punctuation"}},[s._v("}")])]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v(" is my favourite")]),t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")])]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br")])]),t("p",[s._v("你可能不知道的事：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 1. 与引号混用")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" wantToSay "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token template-string"}},[t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v('I\'m a "tbfed"')]),t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")])]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 2. 支持多行文本")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" slogan "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token template-string"}},[t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("\nI have a dream today!\n")]),t("span",{pre:!0,attrs:{class:"token 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\n
...
\n
\n")]),t("span",{pre:!0,attrs:{class:"token template-punctuation string"}},[s._v("`")])]),s._v("\n")])]),s._v(" 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"),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" obj "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("prop")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\nobj"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("prop "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" obj"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("prop "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("+")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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+ MIT 协议 | 版权所有 © 2024-至今 易函123 +
+ + + diff --git a/leetcode/index.html b/leetcode/index.html new file mode 100644 index 0000000..06ab9c8 --- /dev/null +++ b/leetcode/index.html @@ -0,0 +1,85 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 1. leetcode 前言

前端学习，算法与数据结构必不可少。这篇文章，主要是博主刷 leetcod 的相关记录，以及算法与数据结构的学习笔记。该文涉及常用数据结构、排序算法、常用的算法以及前端手写编程题。

主要是为了形成算法思维，养成良好的算法思路。并将相应的数据处理运用到日常开发中。

希望会对大家有些帮助！！

# 2. 笔记内容

# 2.1 复杂度分析

# 2.1.1 时间复杂度

是什么？

  • 一个函数，用大 O 表示，比如 O(1)、O(n)、O(logN)...
  • 定性描述该算法的时间

O(1)

// O(1)
+let i = 0
+i += 1
+
1
2
3

O(n)

// O(n)
+for (let i = 0; i < n; i += 1) {
+  console.log(i)
+}
+
1
2
3
4

O(n)+O(1)=O(n)

// O(n)+O(1)=O(n)
+let i = 0
+i += 1
+for (let j = 0; j < n; j += 1) {
+  console.log(j)
+}
+
1
2
3
4
5
6

O(n)*O(n)=O(n^2)

// O(n)*O(n)=O(n^2)
+for (let i = 0; i < n; i += 1) {
+  for (let j = 0; j < n; j += 1) {
+    console.log(i, j)
+  }
+}
+
1
2
3
4
5
6

O(n)*O(n)=O(n^2)

// O(n)*O(n)=O(n^2)
+for (let i = 0; i < n; i += 1) {
+  for (let j = 0; j < n; j += 1) {
+    console.log(i, j)
+  }
+}
+
1
2
3
4
5
6

O(logN)

// O(logN)
+let i = 1
+while (i > n) {
+  console.log(i)
+  i *= 2
+}
+
1
2
3
4
5
6

# 2.2 常用数据结构

# 2.3 排序算法

# 2.4 查找算法

# 2.5 JavaScript 手写算法

# 2.6 五大常用算法

# 2.7 leetcode 刷题

# 3. 那就开始吧~

写作是一件十分枯燥的事情，如果我写的这些文字对你有些许帮助的话，还请赏个 star 哈~~

Last Updated: 2024-3-25 5:09:19 ├F10: PM┤

+ ← + + 时间复杂度 + + → +

+ + + diff --git a/leetcode/leetcode 0001.html b/leetcode/leetcode 0001.html new file mode 100644 index 0000000..7e720fe --- /dev/null +++ b/leetcode/leetcode 0001.html @@ -0,0 +1,135 @@ + + + + + + leetcode 0001 | Front-end Notes + + + + + + + + +
Front-end Notes

# LeetCode 第 1 号问题：两数之和

# 题目地址

https://leetcode-cn.com/problems/two-sum

# 题目描述

给定一个整数数组 nums 和一个目标值 target，请你在该数组中找出和为目标值的那 两个 整数，并返回他们的数组下标。
+
+你可以假设每种输入只会对应一个答案。但是，数组中同一个元素不能使用两遍。
+
+示例:
+
+给定 nums = [2, 7, 11, 15], target = 9
+
+因为 nums[0] + nums[1] = 2 + 7 = 9
+所以返回 [0, 1]
+
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# 解法一(Map 数据结构)

# 思路

首先把加法转换成减法 +然后利用 Map 的数据结构储存 target 减去某一位置的值和位置 i 对应 +接着判断如果减出的值在 Map 结构中找到则跳出循环，否则将对于的减出来的值和数组的索引 i 对应储存 +最后返回两个索引

# 代码

/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+const twoSum = function (nums, target) {
+  const map = new Map()
+  const len = nums.length //数组长度
+  for (let i = 0; i < len; i++) {
+    //遍历查找数组nums
+    let diff = target - nums[i] //target减去对应的数组的值
+    if (map.has(diff)) {
+      //判断是否已存在对应的减值，存在则取出返回，否则将减值（减出来的值）和对应索引储存
+      return [map.get(diff), i]
+    }
+    map.set(nums[i], i) //将减值（减出来的值）和对应索引储存
+  }
+}
+
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# 解法二(obj 属性)

# 思路

将 obj 替换 Map 去解决问题

# 代码

/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+const twoSum1 = function (nums, target) {
+  const obj = {};
+  const len = nums.length;
+  for (let i = 0; i < len; i++) {
+    let I = nums[i];
+    let j = target - I];
+    if (obj[j] !== undefined) {
+      return [obj[j], i];
+    }
+    obj[I] = i;
+  }
+};
+
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# 解法三(暴力解法)

# 思路

遍历解法

# 代码

/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+const twoSum2 = function (nums, target) {
+  const len = nums.length
+  for (let i = 0; i < len; i++) {
+    for (let j = i + 1; j < len; j++) {
+      if (nums[i] + nums[j] === target) {
+        return [i, j]
+      }
+    }
+  }
+}
+
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# 解法四(map,forEach 的方法，存在问题不会直接返回)

# 思路

需要 try catch，扔出 error 才会返回，

# 代码

/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+const twoSum3 = function (nums, target) {
+  const hash = {}
+  let result = []
+  try {
+    nums.map(function (val, i) {
+      let reduce = target - val
+      if (hash[reduce] !== undefined) {
+        result[0] = hash[reduce]
+        result[1] = i
+        throw 'err'
+      }
+      hash[val] = i
+    })
+  } catch (err) {
+    return result
+  }
+}
+
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Last Updated: 2024-3-27 11:44:51 ├F10: PM┤

+ ← +

+ + + diff --git a/leetcode/space_complexity.html b/leetcode/space_complexity.html new file mode 100644 index 0000000..c84c2c7 --- /dev/null +++ b/leetcode/space_complexity.html @@ -0,0 +1,54 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + + + + + diff --git a/leetcode/time_complexity.html b/leetcode/time_complexity.html new file mode 100644 index 0000000..b6abaaf --- /dev/null +++ b/leetcode/time_complexity.html @@ -0,0 +1,85 @@ + + + + + + 前言 | Front-end Notes + + + + + + + + +
Front-end Notes

# 时间复杂度

# 是什么？

  • 一个函数，用大 O 表示，比如 O(1)、O(n)、O(logN)...
  • 定性描述该算法的时间

O(1)

// O(1)
+let i = 0
+i += 1
+
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O(n)

// O(n)
+for (let i = 0; i < n; i += 1) {
+  console.log(i)
+}
+
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O(n)+O(1)=O(n)

// O(n)+O(1)=O(n)
+let i = 0
+i += 1
+for (let j = 0; j < n; j += 1) {
+  console.log(j)
+}
+
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O(n)*O(n)=O(n^2)

// O(n)*O(n)=O(n^2)
+for (let i = 0; i < n; i += 1) {
+  for (let j = 0; j < n; j += 1) {
+    console.log(i, j)
+  }
+}
+
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O(n)*O(n)=O(n^2)

// O(n)*O(n)=O(n^2)
+for (let i = 0; i < n; i += 1) {
+  for (let j = 0; j < n; j += 1) {
+    console.log(i, j)
+  }
+}
+
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O(logN)

// O(logN)
+let i = 1
+while (i > n) {
+  console.log(i)
+  i *= 2
+}
+
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Last Updated: 2024-3-25 5:09:19 ├F10: PM┤

+ ← + + 空间复杂度 + + → +

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Front-end Notes

# 1. 前言

博主本来可能是一名工地长耕的技术员，机缘巧合之下了解到互联网行业。从土木转行至今，从事前端工作已有七年有余。总觉得工作这么久需要对自己所了解所不解的东西做些体系汇总，建立自己的知识体系。也希望用极简且轻松的语言，来描述对应的知识点。

希望能对大家有所帮助！

# 2. 学习体系规划

  • HTML&CSS:这部分主要是常用布局与盒模型，还有常见面试题汇总。
  • HTML5&CSS3:新特性的使用。
  • JavaScript 基础
  • JavaScript 进阶
  • JavaScript 专题
  • 浏览器的工作原理与实践
  • HTTP 权威指南
  • Webpack/Vite 等构建工具
  • ES6+
  • 算法与数据结构

# 3. 生活小记

生活除了代码还有无数美好的事物值得探索：阳光明媚的早晨、清新的空气、温暖的微风、美味的食物、动人的音乐、感人的电影、真挚的友谊、甜蜜的爱情、令人心动的旅行……

# 4. 那就开始吧

写作是一件十分枯燥的事情，如果我写的这些文字对你有些许帮助的话，还请赏个 star 哈~~

Last Updated: 2024-3-25 11:02:22 ├F10: PM┤

+ 前言 + + → +

+ + +