var vs let vs const in JavaScript
This is the most common JavaScript warm-up question. The differences come down to scope, hoisting, and reassignment — here they are, clearly.
Scope & hoisting
var is function-scoped and hoisted as undefined, so it's readable (as undefined) before its line. let and const are block-scoped and live in the Temporal Dead Zone until declared — accessing them early throws a ReferenceError.
console.log(a); // undefined (var hoisted)
var a = 1;
console.log(b); // ReferenceError (TDZ)
let b = 2;Reassignment
const can't be reassigned — but it locks the binding, not the value. A const object's properties can still be mutated; you just can't point the variable at something new.
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- Does const make an object immutable?
- No. const prevents reassigning the variable, but the object's contents are still mutable. Use Object.freeze for shallow immutability.
- Which should I use?
- const by default, let only when you must reassign, and never var in new code.