FrontendConcepts

`const` vs `let` vs `var`

The difference is the scope

const is constant that cannot be reassigned, it can only be accessed within the block they were declared like let
let is scoped to the nearest enclosing block, which can be smaller than a function block
var is scoped to the nearest function block
Both var and let are global if outside any block

Examples

Global

let me = "go"; // globally scoped
var i = "able"; // globally scoped
console.log(window.me); // undefined
console.log(window.i); // 'able'

Function e.g.1

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
}

Function e.g.2

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // different variable
    console.log(x);  // 2
  }
  console.log(x);  // 1
}`

Redeclaration

"use strict";
let me = "foo";
let me = "bar"; // SyntaxError: Identifier 'me' has already been declared

var me = "foo";
var me = "bar"; // No problem, `me` is replaced.

FrontendConcepts

const vs let vs var

Examples

Reference

`const` vs `let` vs `var`