Scoping

In the old days, we used to always go with the var statement for variable declarations. ES2015 introduces two new declaration variables--let and const. The var statement declares a variable in a function scope:

(function(){ 
    var foo = 1; 
    if ( true ) { 
            var foo = 2; 
            console.log( foo ); 
    } 
    console.log( foo ); 
}()); 

$ node es6.js
2
2

A variable declared with var (foo) spans the entire function scope, meaning that every time we reference it by name, we target the same variable. Both let and const operate on block scopes (if statement, for/while loops, and so on) as shown:

 (function(){ 
    let foo = 1; 
    if ( true ) { 
            let foo = 2; 
            console.log( foo ); 
    } 
    console.log( foo ); 
}()); 

$ node es6.js
2
1

As you can see from the preceding example, we can declare a new variable in a block and it will exist only within that block. The statement const works the same, except it defines a constant that cannot be reassigned after it was declared.