javascript tutorial - [Solved-5 Solutions] Let and Var - javascript - java script - javascript array

The difference is scoping. var is scoped to the nearest function block and let is scoped to the nearest enclosing block, which can be smaller than a function block. Both are global if outside any block.

let me = 'go';  // globally scoped
var i = 'able'; // globally scoped

However, global variables defined with let will not be added as properties on the global windowobject like those defined with var.

console.log(window.me); // undefined
console.log(window.i); // 'able'

Function:

They are identical when used like this in a function block.

function ingWithinEstablishedParameters() {
    let terOfRecommendation = 'awesome worker!'; //function block scoped
    var sityCheerleading = 'go!'; //function block scoped
}

Block:

Here is the difference. let is only visible in the for() loop and var is visible to the whole function.

function allyIlliterate() {
    //tuce is *not* visible out here

    for( let tuce = 0; tuce < 5; tuce++ ) {
        //tuce is only visible in here (and in the for() parentheses)
        //and there is a separate tuce variable for each iteration of the loop
    }

    //tuce is *not* visible out here
}

function byE40() {
    //nish *is* visible out here

    for( var nish = 0; nish < 5; nish++ ) {
        //nish is visible to the whole function
    }

    //nish *is* visible out here
}

Assuming strict mode, var will let you re-declare the same variable in the same scope. On the other hand, let will not:

'use strict';
let me = 'foo';
let me = 'bar'; // SyntaxError: Identifier 'me' has already been declared
'use strict';
var me = 'foo';
var me = 'bar'; // No problem, `me` is replaced.

let can also be used to avoid problems with closures. It binds fresh value rather than keeping an old reference as shown in examples below.

DEMO

for(var i = 1; i < 6; i++) {
  document.getElementById('my-element' + i)
    .addEventListener('click', function() { alert(i) })
}

DEMO (Tested in Chrome and Firefox 50)

'use strict';

for(let i = 1; i < 6; i++) {
  document.getElementById('my-element' + i)
    .addEventListener('click', function() { alert(i) })
}

The accepted answer is missing a point:

{
  let a = 123;
};

console.log(a); // ReferenceError: a is not defined

let is interesting, because it allows us to do something like this:

(() => {
    var count = 0;

    for (let i = 0; i < 2; ++i) {
        for (let i = 0; i < 2; ++i) {
            for (let i = 0; i < 2; ++i) {
                console.log(count++);
            }
        }
    }
})();

(() => {
    var count = 0;

    for (var i = 0; i < 2; ++i) {
        for (var i = 0; i < 2; ++i) {
            for (var i = 0; i < 2; ++i) {
                console.log(count++);
            }
        }
    }
})();

var x = 1;
var x = 2;

let x = 1;
let x = 2;