What is hoisting and how does it affect var, let, function declarations?

Hoisting

Hoisting is JavaScript's behavior of moving declarations to the top of their scope during the compilation phase - before any code runs.

Function declarations - fully hoisted

js
sayHello(); // works fine!

function sayHello() {
  console.log('Hello');
}

The entire function is hoisted - you can call it before it's defined.

var - hoisted but not initialized

js
console.log(name); // undefined (not an error)
var name = 'Alice';
console.log(name); // 'Alice'

// what JavaScript actually sees:
var name;          // declaration hoisted
console.log(name); // undefined
name = 'Alice';    // assignment stays

let and const - temporal dead zone (TDZ)

js
console.log(age); // ReferenceError: Cannot access before initialization
let age = 25;

let and const are hoisted but not initialized - accessing them before declaration throws an error.

Function expressions - not hoisted

js
sayHi(); // TypeError: sayHi is not a function

var sayHi = function() {
  console.log('Hi');
};
// var is hoisted as undefined, calling undefined() throws

Summary

	Hoisted	Initialized	Usable before declaration
`function`	Yes	Yes	Yes
`var`	Yes	`undefined`	Yes (but undefined)
`let` / `const`	Yes	No (TDZ)	No

Hoisting is one of the most common JavaScript interview topics. The key insight is that let and const ARE hoisted - they just aren't usable until the declaration line.

updated Mar 8, 2026

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Hoisting#