LET in ECMAScript 6 - Block Scoped Variables

let has the same block scoping as const, but let us look over Block Scoped Variables in ECMAScript 6

Understanding Block Scope

Let’s compare var and let behavior with this:

    var x = 'global';
    let y = 'global';
    console.log(this.x); //'global'
    console.log(this.y); //undefined

It looks like let variable has its own scope.

    if(1>0){
        var x = 0; //scope is inside the function
        let y = 1; //scope is inside the if-block
    }
    console.log('x:', x); //0
    console.log('y:', y); //undefined

As you can see, var scope is inside the function and let scope is inside if-block.

Why does it happen?

Because when you write code like this:

    function a(){
        if(true){
            var b = 1;
        }
    }

The really thing Javascript doing is :

    function a(){
        var b;
        if(true){
            b = 1;
        }
    }

But when you use let or const it looks another way.

This code:

    function a(){
        let b = 0;
        if(true){
            let b = 1;
        }
     }

Becomes this:

    function a(){
        var b = 1;
        if(true){
            var b$0 = 1;
        }
     }

for loop

It is really not clever, that in JavaScript ES5 when you create a variable in a for loop, you can reach that variable out of that loop.
For example, if you had a variable with the same name it will be rewritten by a variable from inside a for loop.

ECMAScript 5 example:

     var i = 5;
     //...
     for(var i=0; i<10; i++){
        //...
     };
     console.log(i);
     // 10

In ES6 this problem solves with let variable, because it is block-scoped, so our new let variable will be scoped to for block.

ECMAScript 6 example:

      let i = 5;
      //...
      for(let i=0; i<10; i++){
         //...
      };
      console.log(i);
      // 5

let and const best practices

• const variable should become your default variable. (Read about const to know why)
• Use let for simple variables, which are had to be changed.
• It is good to use let in for loops, because you always need block-scoped variable there
• Try to exclude var from your code. var is now the weakest signal available when you define a variable in JavaScript