ES6: The Let Keyword

In ES6, the let keyword was introduced to address some of the issues with the var keyword. Let's take a look at the differences.

First of all, note that the let keyword is considered to be scoped to the block. The examples illustrate this.

Global

When using let in the global scope (in a file outside of functions), it works similar to the var keyword.

var count = 0;
let number = 1;

Yet, when using in a browser, the var will attach to the window, but not the let.

window.count // equals 0
window.number // undefined

Function

let and var work the same inside a function.

function myCoolFunction() {
  let myLetVariable = 'woot';
  var myLetVariable = 'woot, woot';
}

Block or For Loops

The big difference is when using var and let inside of a For Loop.

functon forLoopWithLet() {
  // can't access count

  for (let count = 0; count < 10; count += 1) {
    // can access count and each count is a new variable
  }

  // can't access count
}

functon forLoopWithVar() {
  // you can access count

  for (var count = 0; count < 10; count += 1) {
    // can access count and each count is the same variable
  }

  // you can access count
}

Declaring

The final difference is that var variables can be redeclared, but let can not.

var one = 'one';
var one = 'two'; // Works

let one = 'one';
let one = 'two'; // Syntax Error