Services
Technology Web App Development Mobile App Development Windows App Development Database Development UI/UX Development Cloud Development SEO Digital Marketing Guest Posting Content Writing Artificial Intelligence Data Science ERP Development CRM Development CMS Development LMS Development Business Listing Blockchain Technology Startup Promotion AI Tools Integration
Products
Data Converter Cleaner TUC - Unit Converter Import Export Survey Manager Tweet Controller
Developer
Article Blog Forum Interview Beginner News Startup Quiz
Users
Career
Apply Job Internship
Pricing
Login Sign Up Get a Quote

interview

Home Interviews Explain the concept of "hoisting" in JS. How does it affect variable and function declarations? – MindStick
Ask Question

1 Answers

Ravi Vishwakarma
Ravi Vishwakarma 17 Jun 2024
Report Abuse

JavaScript Hoisting refers to the process whereby the interpreter appears to move the declaration of functions, variables, classes, or imports to the top of their scope, before execution of the code.

Variable Hoisting:

When you declare a variable using var or let, the declaration is hoisted to the top of its scope. Here's how hoisting affects variables:

  • Declaration Hoisting: The declaration of a variable using the var or let keyword is hoisted to the top of its containing scope. This means the variable is available throughout the scope, even before the actual line where it's declared.
console.log(x); // undefined (not ReferenceError) due to hoisting
var x = 10;

After hoisting, the code behaves like this:

var x; // declaration is hoisted
console.log(x); // undefined
x = 10; // initialization happens at its original position
  • Initialization: Only the declaration (not the initialization) is hoisted. Therefore, if you try to access the value of a variable before it is initialized, you will get "undefined".
console.log(x); // undefined
var x = 10;
  • Block Scope with let: Variables declared with let and const are also hoisted to the top of their block scope, but they are not initialized. Trying to access them before their declaration results in a 'ReferenceError'.
console.log(x); // ReferenceError: Cannot access 'x' before initialization
let x = 10;

Function Hoisting:

Function declarations are fully hoisted, including both the function name and its body. This allows you to call a function before its actual declaration in the code.

  • Function Declaration Hoisting: 
foo(); // 'Hello, world!'

function foo() {
    console.log('Hello, world!');
}

After hoisting code like this 

function foo() {
    console.log('Hello, world!');
}
foo(); // 'Hello, world!'
  • Function Expressions are Not Hoisted: Function expressions (where you assign a function to a variable) are not hoisted in the same way as function declarations. Only the variable declaration is hoisted, not the function assignment.
foo(); // TypeError: foo is not a function

var foo = function() {
    console.log('Hello, world!');
};

After hoisting code behaves like this.

var foo;
foo(); // foo is not a function 
foo = function() {
    console.log('Hello, world!');
};
MindStick Training
Markdown for AI

A clean, structured version of this page for AI assistants and LLMs.

Open .md