JavaScript - Hoisting

Overview

Estimated time: 15–20 minutes

Hoisting moves declarations to the top of their scope. var/function are hoisted differently than let/const.

Learning Objectives

• Explain hoisting behavior for var and function declarations.
• Recognize the temporal dead zone (TDZ) for let/const.

Prerequisites

• JavaScript - Variable Scope

Examples

console.log(a); // undefined (declaration hoisted)
var a = 10;

// console.log(b); // ReferenceError (TDZ)
let b = 20;

foo(); // works (function declaration hoisted)
function foo(){ console.log('ok'); }

Notes

• Function declarations are hoisted with their bodies.
• let/const are hoisted but uninitialized until declaration (TDZ).