What hoisting is in Javascript

Introduction

Hoisting helps you understand how JavaScript moves some code parts before running. It explains why you can use some things before you write them.

When you want to know why a variable or function works even if declared later in the code.

When debugging errors about variables or functions not found.

When learning how JavaScript reads your code step by step.

When writing functions and variables to avoid unexpected behavior.

When explaining to others why some code runs without errors even if order looks wrong.

Syntax

Javascript

// Variables and functions are 'hoisted' to the top
console.log(name); // undefined
var name = 'Alice';

sayHi();
function sayHi() {
  console.log('Hi!');
}

Only declarations are hoisted, not initializations.

Function declarations are fully hoisted, variables declared with var are hoisted but not their values.

Examples

The variable age is hoisted but its value is not, so it prints undefined.

Javascript

console.log(age); // undefined
var age = 25;

The function greet is hoisted completely, so it can be called before its definition.

Javascript

greet();
function greet() {
  console.log('Hello!');
}

Variables declared with let are hoisted but are in a temporal dead zone until initialized, so this causes a ReferenceError.

Javascript

console.log(city); // ReferenceError
let city = 'Paris';

Sample Program

This program shows how var variables are hoisted with undefined value, and functions are fully hoisted.

Javascript

console.log(message); // What will this print?
var message = 'Hello, world!';

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

Important Notes

Hoisting only moves declarations, not assignments.

Use let and const to avoid confusion from hoisting.

Function expressions are not hoisted like function declarations.

Summary

Hoisting means JavaScript moves declarations to the top before running code.

Variables declared with var are hoisted but start as undefined.

Functions declared with function are fully hoisted and can be called before they appear.