How do let and const differ from var?

JavaScriptJunior~1 min read

1. Main differences

Criterion	`var`	`let`	`const`
Scope	Function or global	Block (`{ ... }`)	Block (`{ ... }`)
Hoisting	Hoisted and initialized to `undefined`	Hoisted, but not initialized (error if accessed before declaration)	Hoisted, but not initialized (error if accessed before declaration)
Re-declaration	Allowed within the same scope	Error	Error
Reassigning the value	Allowed	Allowed	Not allowed
Added to the window object (in the browser)	Yes (`window.varName`)	No	No

2. Scope

`var` - function scope

javascript

if (true) {
  var a = 10;
}
console.log(a); // 10 (accessible outside the block!)

`let` / `const` - block scope

javascript

if (true) {
  let b = 20;
  const c = 30;
}
console.log(b); // ReferenceError
console.log(c); // ReferenceError

let and const only "live" inside a {} block.

3. Hoisting (variable hoisting)

All variables are "hoisted" to the top of their scope, but var is initialized with the value undefined, while let and const fall into the Temporal Dead Zone (TDZ).

javascript

console.log(a); // undefined (var is hoisted)
var a = 5;

console.log(b); // ReferenceError (in the TDZ)
let b = 10;

console.log(c); // ReferenceError (in the TDZ)
const c = 15;

4. Re-declaration and reassignment

javascript

var x = 1;
var x = 2; // allowed
console.log(x); // 2

let y = 1;
let y = 2; // SyntaxError: Identifier 'y' has already been declared

const z = 1;
z = 2; // TypeError: Assignment to constant variable

5. Behavior with objects and arrays under `const`

const protects the reference, not the contents.

javascript

const user = { name: 'Tim' };
user.name = 'Alex'; // the property can be changed
console.log(user.name); // 'Alex'

user = {}; // The object itself cannot be reassigned

6. Behavior in the browser (global variables)

javascript

var x = 10;
let y = 20;
const z = 30;

console.log(window.x); // 10
console.log(window.y); // undefined
console.log(window.z); // undefined

var adds the variable to the global object (window, globalThis), while let and const do not, which makes the code safer.

7. Summary - how to use them

Situation	What to use	Why
The variable needs to change	`let`	Safer than `var`, with block scope
The variable should not change	`const`	Explicitly shows immutability
Legacy code (pre-ES6)	`var`	Outdated, use only to support old projects

A simple rule of thumb

const - by default. let - when you need to change the value. var - never use it in modern code.

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