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What is hoisting in JavaScript?
Declarations are registered during the creation phase — before code runs line by line.
Ideal Answer
Hoisting is the behavior where the engine registers declarations during the creation phase of an execution context, before the code runs line by line. It is not literally moving lines of source code.
| Declaration | Hoisted? | Usable before line? |
|---|---|---|
function declaration |
Yes — name and body | Yes |
var |
Yes — initialized to undefined |
Yes (value is undefined) |
let / const |
Yes — but in TDZ | No — ReferenceError |
class |
Yes — but in TDZ | No — ReferenceError |
function expression (const fn = function() {}) |
Variable only | No — TDZ until assignment |
JavaScript
sayHi() // works — function declaration fully hoisted
function sayHi() { console.log('hi') }
console.log(x) // undefined — var hoisted, not assigned yet
var x = 1
console.log(y) // ReferenceError — TDZ
let y = 2Interview nuance: let/const are hoisted to the top of their block, but unlike var they are not initialized until the declaration executes — that gap is the temporal dead zone.