In upcoming JavaScript ES6 (EcmaScript 2015), they introduce new way to define variables by using let and const. As you may be know as JavaScript developer, we are already familiar with var. Let's inspect them more.

Let

Unlike var which defines variable globally or locally to an entire function, let creates variables that are limited in scope to the block, statement or expression.

Check this example below

// Var
function testVar() {
  var x = 10;

  if (true) {
    var x = 50;
    console.log(x); // print 50
  }

  console.log(x); // print 50
}

// Let
function testLet() {
  let x = 10;

  if (true) {
    let x = 50;
    console.log(x); // print 50
  }

  console.log(x); // print 10
}

In let example, we can see that variable x inside if statement is block scoped. That's why it prints 50 instead of 10.

Const

Const main purpose is for creating read-only reference to a value but the value itself is mutable. You can say that it is const reference not const value. That is the reason why const variable can't be reassigned.

Check this out example below

const name = "bugan"; 
name = "diana";

console.log(name); // bugan

In spite of giving new assignment, the value of name is still "bugan". Let's try with array example.

const fruits = ["apple", "mango"]; 
fruits.push("kiwi");

console.log(fruits); // ["apple", "mango", "kiwi"]

fruits = ["orange"]; 
console.log(fruits); // ["apple", "mango", "kiwi"]

As you can see, although we define fruits as const, the value itself is mutable (we added kiwi). However, when we reassign fruits to another value, it doesn't work.