const-var-let.md

let vs var

var is function scoped and if you try to use a variable declared with var before the actual declaration, you’ll just get undefined. const and let are blocked scoped and if you try to use variable declared with let or const before the declaration you’ll get a "ReferenceError variable is not defined".

function discountPrices(prices, discount) {
  var discounted = []

  for (var i = 0; i < prices.length; i++) {
    var discountedPrice = prices[i] * (1 - discount)
    var finalPrice = Math.round(discountedPrice * 100) / 100
    discounted.push(finalPrice)
  }

  console.log(i) // 3
  console.log(discountedPrice) // 150
  console.log(finalPrice) // 150

  return discounted
}

Take a look at that for loop. Are the variables declared inside of it accessible outside of it? Turns out, they are. since variables declared with var are function scoped

Now do the same function with let

function discountPrices(prices, discount) {
  let discounted = []

  for (let i = 0; i < prices.length; i++) {
    let discountedPrice = prices[i] * (1 - discount)
    let finalPrice = Math.round(discountedPrice * 100) / 100
    discounted.push(finalPrice)
  }

  console.log(i) // ReferenceError: i is not defined
  console.log(discountedPrice)
  console.log(finalPrice)

  return discounted
}

discountPrices([100, 200, 300], 0.5) // ReferenceError: i is not defined

We get ReferenceError: i is not defined. What this tells us is that variables declared with let are block scoped, not function scoped. So trying to access i (or discountedPrice or finalPrice) outside of the “block” they were declared in is going to give us a reference error as we just barely saw.

const - In constants mutation is allowed but reassignment is not allowed.

const person = { name: 'Kim Kardashian' }

person.name = 'Kim Kardashian West' // ✅

person = {} // ❌ Assignment to constant variable.

Notice that changing a property on an object isn’t reassigning it, so even though an object is declared with const, that doesn’t mean you can’t mutate any of its properties. It only means you can’t reassign it to a new value.

Very Important - There is another great advantage using let as it creates a new lexical environment and also binds fresh value rather than keeping an old reference.

for (var i = 1; i < 6; i++) {
  setTimeout(function() {
    console.log(i)
  }, 1000)
}

// Now the same with 'let'

for (let i = 1; i < 6; i++) {
  setTimeout(function() {
    console.log(i)
  }, 1000)
}

The first for loop will print the last value 5 times, with let it creates a new scope and bind fresh values printing us 1, 2, 3, 4, 5.

Difference between Object.freeze() and const

const and Object.freeze are two completely different things.

const applies to bindings ("variables"). It creates an immutable binding, i.e. you cannot assign a new value to the binding.

Object.freeze works on values, and more specifically, object values. Object.freeze() is a method which accepts an object and returns the same object. Now the object cannot have any of its properties removed or any new properties added. It makes an object immutable, i.e. you cannot change its properties.

On the other hand if I use const to declare and object, it doesn't "freeze" them, you just can't redeclare the whole object, but you can modify its keys freely. On the other hand you can redeclare frozen objects.

const obj = {
  name: "rohan",
}

obj.name = "paul"

console.log(obj)