What is the difference between var, let, and const in JavaScript?

var is function-scoped and hoisted; let and const are block-scoped. Use const by default, let when you need to reassign, and avoid var.

JavaScript

Overview

`var`, `let`, and `const` all declare variables, but they differ in scope, hoisting behaviour, and reassignability. `let` and `const` were introduced in ES2015 to fix the surprising behaviour of `var`.

Key Points

•`var` is function-scoped (or global), hoisted to the top of its function, and can be re-declared.
•`let` is block-scoped `{}`, not accessible before declaration (temporal dead zone), and can be reassigned.
•`const` is block-scoped, cannot be reassigned, but object/array contents can still be mutated.
•Use `const` by default; use `let` only when you need to reassign. Never use `var` in new code.
•Hoisting: `var` declarations are moved to the top and initialised as `undefined`; `let`/`const` are hoisted but not initialised.

Examples

Scope difference: var leaks out of blocks

javascript

if (true) {
  var x = 'var';   // accessible outside the block
  let y = 'let';   // block-scoped, only inside {}
}
console.log(x); // 'var'
console.log(y); // ReferenceError: y is not defined

const prevents reassignment, not mutation

javascript

const user = { name: 'Alice' };
user.name = 'Bob';  // OK mutating the object
user = {};          // TypeError: Assignment to constant variable

The var hoisting trap

javascript

console.log(a); // undefined (not an error var is hoisted)
var a = 5;

console.log(b); // ReferenceError temporal dead zone
let b = 5;

Pro Tip

Default to `const`. If you get an error about reassignment, switch that one declaration to `let`. This makes your intent clear: `const` means "this binding never changes", `let` means "this will change".