Javascript - “let” keyword vs “var” keyword

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

Global:

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

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

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)
    };

    //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
};