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.

Practice

1. What does hoisting mean in JavaScript?

easy

A. JavaScript converts all variables to global scope automatically.

B. JavaScript moves variable and function declarations to the top before running code.

C. JavaScript delays execution of code until user interaction.

D. JavaScript runs code only after the entire file is loaded.

Solution

1. Step 1: Understand hoisting concept

   Hoisting means JavaScript moves declarations of variables and functions to the top of their scope before running the code.

2. Step 2: Compare options

   Only JavaScript moves variable and function declarations to the top before running code. correctly describes this behavior. Other options describe unrelated behaviors.

3. Final Answer:

   JavaScript moves variable and function declarations to the top before running code. -> Option B

4. Quick Check:

   Hoisting = Moving declarations up [OK]

Hint: Hoisting means declarations move up before code runs [OK]

Common Mistakes:

• Thinking hoisting moves variable assignments, not just declarations
• Confusing hoisting with asynchronous code execution
• Believing hoisting applies to all variable types equally

2. Which of the following variable declarations is hoisted and initialized as undefined?

easy

A. function foo() {}

B. const y = 10;

C. let x = 5;

D. var z = 15;

Solution

1. Step 1: Identify hoisting behavior for variable types

   Variables declared with var are hoisted and initialized as undefined. let and const are hoisted but not initialized, causing a temporal dead zone.

2. Step 2: Match options to behavior

   var z = 15; uses var, so it is hoisted and initialized as undefined. Others do not behave this way.

3. Final Answer:

   var z = 15; -> Option D

4. Quick Check:

   var hoisted as undefined [OK]

Hint: Only var variables are hoisted and set to undefined [OK]

Common Mistakes:

• Assuming let and const behave like var with hoisting
• Confusing function declarations with variable declarations
• Thinking function expressions are hoisted like functions

3. What will be the output of this code?

console.log(myVar);
var myVar = 10;

medium

A. undefined

B. 10

C. ReferenceError

D. null

Solution

1. Step 1: Understand var hoisting with initialization

   The declaration var myVar is hoisted to the top, but the assignment = 10 stays in place.

2. Step 2: Evaluate console.log output

   At the time of console.log(myVar), myVar exists but is undefined because assignment hasn't happened yet.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   var declaration hoisted, value undefined [OK]

Hint: var declaration hoisted, value assigned later [OK]

Common Mistakes:

• Expecting the value 10 to print immediately
• Confusing ReferenceError with undefined for var
• Thinking let or const behave the same way

4. Find the error in this code related to hoisting:

console.log(foo());
var foo = function() { return 'Hello'; };

medium

A. No error, outputs 'Hello'

B. SyntaxError

C. TypeError: foo is not a function

D. ReferenceError: foo is not defined

Solution

1. Step 1: Understand hoisting of function expressions

   Variable foo is hoisted as undefined, but the function assignment is not hoisted.

2. Step 2: Analyze calling foo before assignment

   Calling foo() before assignment tries to call undefined as a function, causing a TypeError.

3. Final Answer:

   TypeError: foo is not a function -> Option C

4. Quick Check:

   Function expressions not hoisted as functions [OK]

Hint: Function expressions hoisted as undefined, not callable [OK]

Common Mistakes:

• Thinking function expressions are fully hoisted
• Expecting no error because of hoisting
• Confusing ReferenceError with TypeError

5. Consider this code:

console.log(greet());

function greet() {
  return 'Hi!';
}

var greet = function() {
  return 'Hello!';
};

console.log(greet());

What will be the output?

hard

A. 'Hi!' then ReferenceError

B. 'Hello!' then 'Hello!'

C. ReferenceError then 'Hello!'

D. 'Hi!' then 'Hello!'

Solution

1. Step 1: Analyze hoisting of function declaration

   The function declaration function greet() is fully hoisted, so the first greet() call returns 'Hi!'.

2. Step 2: Analyze var assignment after first call

   The var greet declaration is hoisted but initialized as undefined. The assignment to a function expression happens after the first call, so the second greet() call uses the new function returning 'Hello!'.

3. Final Answer:

   'Hi!' then ReferenceError -> Option A

4. Quick Check:

   Function declaration hoisted, var assignment overrides later [OK]

Hint: Function declarations hoisted fully; var assignments override later [OK]

Common Mistakes:

• Thinking var assignment overrides before first call
• Expecting ReferenceError due to var redeclaration
• Confusing function declaration and expression hoisting