ES6 introduced two new kinds of variables in Javascript, const and let. They are block scoped variables, meaning they are scope inside {}, where var is hoisted to the global or functional scope.

const vs let vs var

Let’s compare some small code snippets and their outputs:

Snippet 1 - for loop

// var
// console.log(i); // undefined
for (var i = 0; i < 3; i++){
  console.log(i); // 0; 1; 2
}
console.log(i); // 3

//-----------------

// let
// console.log(i); // ReferenceError: i is not defined
for (let i = 0; i < 3; i++){
  console.log(i); // 0; 1; 2
}
console.log(i); // ReferenceError: i is not defined

//-----------------

// const
// console.log(i); // ReferenceError: i is not defined
for (const i = 0; i < 3; i++){
  console.log(i); // 0; ReferenceError: i is not defined
}
console.log(i);

This first code snippet shows us that const and let are scoped at a block level, where var is scoped and hoisted, here in the global scope.

We can see as well that the value of const cannot change in our for loop.

Snippet 2 - Reassign values

// var
var test = 1;
console.log(test); // 1
test = 2;
console.log(test); // 2

//-----------------

// let
let test = 1;
console.log(test); // 1
test = 2;
console.log(test); // 2

//-----------------

// const
const test = 1;
console.log(test); // 1
test = 2;
console.log(test); // TypeError: invalid assignment to const `test'

This example shows us that the value of const can’t be reassigned.

Snippet 3 - Redeclare variable

// var
var test = 1;
console.log(test); // 1
var test = 2;
console.log(test); // 2

//-----------------

// let
let test = 1;
console.log(test); // SyntaxError: redeclaration of let test
let test = 2;
console.log(test);

//-----------------

// const
const test = 1;
console.log(test); // SyntaxError: redeclaration of const test
const test = 2;
console.log(test);

Both let and const can’t be redeclare

Snippet 4 - Scope

// var
var test = 1;
function testFunction() {
  var test = 2;
  console.log(test); // 2
}
console.log(test); // 1

//-----------------

// let
let test = 1;
if(test > 0) {
  let test = 2;
  console.log(test); // 2
}
console.log(test); // 1

//-----------------

// const
const test = 1;
if(test > 0) {
  const test = 2;
  console.log(test); // 2
}
console.log(test); // 1

The above examples, show us that using let and const can reuse in another scope.

Snippet 5 - Object

// var
var obj = {
  test1: 1,
  test2: 2
};
// obj = {test: 0};
// console.log(obj); // { test: 0 }
obj.test1 = 10;
console.log(obj); // { test1: 10, test2: 2 }

//-----------------

// let
let obj = {
  test1: 1,
  test2: 2
};
// obj = {test: 0};
// console.log(obj); // { test: 0 }
obj.test1 = 10;
console.log(obj); // { test1: 10, test2: 2 }

//-----------------

// const
const obj = {
  test1: 1,
  test2: 2
};
// obj = {test: 0};
// console.log(obj); // invalid assignment to const `obj'
obj.test1 = 10;
console.log(obj); // { test1: 10, test2: 2 }

This last example shows us that with const we can mutate properties in our variable.

const & let support

const

Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can’t be redeclared. MDN - const

Support

Can I Use const? Data on support for the const feature across the major browsers from caniuse.com.

let

The let statement declares a block scope local variable, optionally initializing it to a value. MDN - let

Support

Can I Use let? Data on support for the let feature across the major browsers from caniuse.com.

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