What is Hoisting in JavaScript?

What is Hoisting?

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope before code executes.

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var Hoisting

var declarations are hoisted, but initializations are not:

console.log(a); // undefined (no error)
var a = 5;
console.log(a); // 5

// Equivalent to:
var a;           // hoisted to top
console.log(a); // undefined
a = 5;

let and const — Temporal Dead Zone (TDZ)

let and const are hoisted but cannot be accessed before declaration — this is the Temporal Dead Zone:

console.log(b); // ReferenceError: Cannot access 'b' before initialization
let b = 10;

Function Declaration Hoisting

Function declarations are fully hoisted (including their body):

foo(); // ✅ 'Hello' (callable before declaration)
function foo() {
  console.log('Hello');
}

Function Expressions Are NOT Hoisted

bar(); // ❌ TypeError: bar is not a function
var bar = function() {
  console.log('World');
};

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Summary

Declaration	Hoisted	Initial Value	TDZ
var	✅	undefined	❌
let	✅	uninitialized	✅
const	✅	uninitialized	✅
Function decl.	✅	Full function	❌
Function expr.	Variable only	undefined	—