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ECMA6 - 读书笔记
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<h1>Let 和 Const</h1>
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<p>let不允许在相同作用域内，重复声明同一个变量。</p>
<p>ES6 明确规定，如果区块中存在<strong>let</strong>和<strong>const</strong>命令，这个区块对这些命令声明的变量，从一开始就形成了封闭作用域。凡是在声明之前就使用这些变量，就会报错。</p>
<p>ES6 规定暂时性死区和let、const语句不出现变量提升，主要是为了减少运行时错误，防止在变量声明前就使用这个变量，从而导致意料之外的行为。这样的错误在 ES5 是很常见的，现在有了这种规定，避免此类错误就很容易了。</p>
<p>ES5 只有全局作用域和函数作用域，没有块级作用域，实际上使得获得广泛应用的立即执行函数表达式（IIFE）不再必要了。<br/>
ES5 规定，函数只能在顶层作用域和函数作用域之中声明，不能在块级作用域声明。<br/>
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const声明一个只读的常量。一旦声明，常量的值就不能改变。</p>
<p>const实际上保证的，并不是变量的值不得改动，而是变量指向的那个内存地址不得改动。对于简单类型的数据（数值、字符串、布尔值），值就保存在变量指向的那个内存地址，因此等同于常量。但对于复合类型的数据（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固定的，至于它指向的数据结构是不是可变的，就完全不能控制了。因此，将一个对象声明为常量必须非常小心。</p>
<p>顶层对象，在浏览器环境指的是window对象，在 Node 指的是global对象。ES5 之中，顶层对象的属性与全局变量是等价的。</p>
<p>ES6 为了改变这一点，一方面规定，为了保持兼容性，var命令和function命令声明的全局变量，依旧是顶层对象的属性；另一方面规定，let命令、const命令、class命令声明的全局变量，不属于顶层对象的属性。也就是说，从 ES6 开始，全局变量将逐步与顶层对象的属性脱钩。</p>
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<span class="date">2018/1/9</span>
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