JavaScript Interview Questions
JavaScript is one of the most popular programming languages in the world. Whether you’re a seasoned developer or a beginner looking to land your first job, preparing for JavaScript interviews is crucial. This article provides an extensive list of 200 JavaScript interview questions, along with detailed answers and examples, to help you ace your next interview.
Basic JavaScript Questions
1. What is JavaScript?
JavaScript is a lightweight, interpreted programming language with first-class functions, most known as the scripting language for Web pages, but it’s used in many non-browser environments as well.
2. What are the different data types present in JavaScript?
JavaScript supports six primitive data types:
- Undefined: A variable that has not been assigned a value.
- Null: A variable that is explicitly set to null.
- Boolean: Represents true or false values.
- Number: Represents both integer and floating-point numbers.
- String: Represents sequence of characters.
- Symbol: A unique and immutable primitive value.
Example:
let name = "John"; // String
let age = 30; // Number
let isMarried = false; // Boolean
let height; // Undefined
let nothing = null; // Null
let uniqueID = Symbol("id"); // Symbol
3. What is the difference between == and ===?
The == operator compares values for equality, after converting both values to a common type. The === operator compares both value and type without performing type conversion.
Example:
console.log(5 == '5'); // true
console.log(5 === '5'); // false
4. Explain Hoisting in JavaScript.
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase. Only declarations are hoisted, not initializations.
Example:
console.log(x); // undefined
var x = 5;
5. What is the difference between let, var, and const?
- var: Function-scoped or globally-scoped, can be redeclared and updated.
- let: Block-scoped, cannot be redeclared within the same scope, but can be updated.
- const: Block-scoped, cannot be redeclared or updated within the same scope.
Example:
var a = 10;let b = 20;const c = 30;
Intermediate JavaScript Questions
6. What is an Immediately Invoked Function Expression (IIFE)?
An IIFE is a function that is executed immediately after it is defined. It has no name and is not accessible after its initial execution.
Example:
(function() { console.log("IIFE executed!");})();
7. Explain Closures in JavaScript.
A closure is a function that remembers its outer variables and can access them. Closures are created every time a function is created, at function creation time.
Example:
function outerFunction() {
let outerVariable = "I'm outside!";
function innerFunction() { console.log(outerVariable); }
return innerFunction;
}const inner = outerFunction();inner(); // I'm outside!
8. What is the purpose of the bind method?
The bind method creates a new function that, when called, has its this keyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
Example:
const person = { firstName: "John", lastName: "Doe", fullName: function() { return this.firstName + " " + this.lastName;
}};const member = {
firstName: "Hege", lastName: "Nilsen"
};let fullName = person.fullName.bind(member);console.log(fullName()); // Hege Nilsen
9. What are Promises in JavaScript?
A Promise is an object representing the eventual completion or failure of an asynchronous operation. It can be in one of three states: pending, fulfilled, or rejected.
Example:
let promise = new Promise((resolve, reject) => { let success = true; if (success) { resolve("Promise fulfilled!"); } else { reject("Promise rejected.");
}});promise.then( result => console.log(result), error => console.log(error)
);
10. What is the difference between call and apply?
The call method calls a function with a given this value and arguments provided individually. The apply method calls a function with a given this value, and arguments provided as an array (or an array-like object).
Example:
function greet(greeting, punctuation) { console.log(greeting + ', ' + this.name + punctuation);}
const person = {name: 'John'};greet.call(person, 'Hello', '!'); // Hello,John!
greet.apply(person, ['Hi', '.']); // Hi, John.
Advanced JavaScript Questions
11. What are JavaScript Prototypes?
Every JavaScript object has a prototype. A prototype is also an object. All JavaScript objects inherit their properties and methods from their prototype.
Example:
function Person(firstName, lastName) {
this.firstName = firstName; this.lastName = lastName;}Person.prototype.fullName = function() { return this.firstName + ' ' + this.lastName;};let person1 = new Person('John', 'Doe');console.log(person1.fullName()); // John Doe
12. What is the Event Loop in JavaScript?
The Event Loop is a mechanism that handles the execution of multiple pieces of code, allowing JavaScript to perform non-blocking asynchronous operations. It continuously checks the call stack and the callback queue, pushing the first callback in the queue to the call stack if the call stack is empty.
13. Explain the concept of this keyword in JavaScript.
The this keyword refers to the object it belongs to. Its value is determined by how a function is called:
- In a method,
thisrefers to the owner object. - In a function,
thisrefers to the global object (orundefinedin strict mode). - In an event,
thisrefers to the element that received the event.
Example:
const person = { firstName: "John",
lastName: "Doe", fullName: function() { return this.firstName + " " + this.lastName; }};console.log(person.fullName()); // John Doe
14. What is Asynchronous JavaScript?
Asynchronous JavaScript refers to code that doesn’t execute linearly from top to bottom. Instead, it can execute other tasks while waiting for an operation to complete, typically using callbacks, promises, or async/await.
15. Explain the concept of Debouncing in JavaScript.
Debouncing is a programming practice used to ensure that time-consuming tasks do not fire so often, reducing the number of times a function is called. It limits the rate at which a function is executed.
Example:
function debounce(func, delay) { let debounceTimer; return function() {
const context = this; const args = arguments;
clearTimeout(debounceTimer); debounceTimer = setTimeout(() => func.apply(context, args), delay); };}window.addEventListener('resize', debounce(() => { console.log('Resize event debounced!');
},300));JavaScript ES6+ Questions
16. What are Arrow Functions?
Arrow Functions are a concise way to write functions in JavaScript. They do not have their own this and are always anonymous.
Example:
const add = (a, b) => a + b;console.log(add(2, 3)); // 5
17. What is Destructuring in JavaScript?
Destructuring is a syntax that allows you to assign properties of an array or object to variables using a syntax that looks similar to array or object literals.
Example:
const person = {name: 'John', age: 25};const {name, age} = person;console.log(name); // John
console.log(age); // 25
const numbers = [1, 2, 3];
const [a, b, c] = numbers;
console.log(a, b, c); // 1 2 3
18. What are Template Literals?
Template Literals are string literals allowing embedded expressions. They are enclosed by backticks () and can contain placeholders indicated by the dollar sign and curly braces ${expression}`.
Example:
let name = "John";let greeting = `Hello, ${name}!`;console.log(greeting); // Hello, John!19. What are Default Parameters in JavaScript?
Default Parameters allow you to initialize named parameters with default values if no value or undefined is passed.
Example:
function greet(name = 'Guest') { console.log(`Hello, ${name}!`);}greet('John'); // Hello, John!
greet(); // Hello, Guest!
20. Explain the async and await keywords in JavaScript.
The async function declaration defines an asynchronous function, which returns an AsyncFunction object. The await keyword is used to wait for a Promise. It can only be used inside an async function.
Example:
async function fetchData() { let response = await fetch('https://api.example.com/data'); let data = await response.json(); return data;}
fetchData().then(data => console.log(data));
JavaScript Interview Question Examples
21. What will the following code output?
console.log(1 + "2" + "2");
Answer: "122". In JavaScript, the + operator is used for both addition and concatenation. When a string is involved, it concatenates the values.
22. How would you remove duplicate values from an array?
const numbers = [1, 2, 3, 3, 4, 5, 5];const uniqueNumbers = Array.from(new Set(numbers));console.log(uniqueNumbers); // [1, 2, 3, 4, 5]
23. Explain the concept of event delegation.
Event delegation is a technique involving adding a single event listener to a parent element, rather than adding multiple listeners to child elements. This can improve performance and manage dynamically added elements.
24. What is the difference between null and undefined?
nullis a value explicitly assigned by a developer to indicate no value or empty object.undefinedis a variable that has been declared but not assigned a value.
25. How would you reverse a string in JavaScript?
const str = "Hello, World!";const reversedStr = str.split('').reverse().join('');console.log(reversedStr); // "!dlroW ,olleH"
26. Explain the concept of prototypal inheritance in JavaScript.
Prototypal inheritance is the mechanism by which objects in JavaScript inherit features from one another. Each object has a prototype object, and properties and methods are inherited from this prototype object.
27. What are modules in JavaScript?
Modules are reusable pieces of code that can be exported from one program and imported into another program. This allows for better code organization and reduces the risk of naming conflicts.
28. How would you check if a variable is an array in JavaScript?
const arr = [1, 2, 3];console.log(Array.isArray(arr)); // true
29. Explain the concept of the spread operator (...) in JavaScript.
The spread operator allows an iterable to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected.
Example:
const numbers = [1, 2, 3];
const newNumbers = [...numbers, 4, 5];
console.log(newNumbers); // [1, 2, 3, 4, 5]
30. How would you sort an array of objects by a specific property value?
const persons = [
{ name: 'John', age: 30 }, { name: 'Jane', age: 25 }, { name: 'Doe', age: 40 }];persons.sort((a, b) => a.age - b.age);console.log(persons); // Sorted by age ascending
Conclusion
Mastering JavaScript is essential for anyone looking to excel in web development and programming interviews. This comprehensive list of 200 JavaScript interview questions covers a wide range of topics, from basic concepts to advanced techniques. By preparing with these questions, along with understanding their answers and examples, you’ll be well-equipped to tackle any JavaScript interview with confidence.
FAQ: Frequently Asked Questions about JavaScript
1. What is JavaScript?
JavaScript is a high-level, interpreted programming language primarily used for making web pages interactive. It runs in web browsers and allows developers to create dynamic content and control multimedia elements.
2. Why should I learn JavaScript?
Learning JavaScript is essential for web development because:
- It enhances user experience by making websites interactive.
- It’s versatile and widely used across front-end, back-end, and mobile app development.
- It has a large and active developer community, providing extensive resources and libraries.
3. What are the different data types in JavaScript?
JavaScript supports several data types:
- Primitive types: Number, String, Boolean, Undefined, Null, Symbol.
- Non-primitive types: Object (including Arrays and Functions).
4. What are the key features of JavaScript?
Some key features of JavaScript include:
- Dynamic typing: Variables can hold values of any type without any type enforcement.
- Prototypal inheritance: Objects can inherit properties and methods from other objects.
- Functions as first-class citizens: Functions can be passed as arguments and returned from other functions.
5. How does JavaScript differ from Java?
Despite the similar names, JavaScript and Java are different:
- Purpose: JavaScript is primarily used for client-side scripting in web browsers, while Java is a general-purpose language used for building applications.
- Syntax and Structure: JavaScript has C-style syntax, while Java syntax is similar to C++.
6. What are closures in JavaScript?
Closures are functions that retain access to variables from the scope in which they were defined, even after the parent function has finished executing. They are often used to create private variables and encapsulate functionality.
7. How does asynchronous programming work in JavaScript?
JavaScript uses callbacks, Promises, and async/await to handle asynchronous operations. This allows tasks like fetching data from servers or handling user events to occur without blocking the main thread.
8. What are Arrow Functions in JavaScript?
Arrow functions are a concise syntax for writing function expressions in JavaScript. They provide a more compact and readable way to write functions, especially when using functions as arguments or in callbacks.
9. How can I debug JavaScript code effectively?
Debugging JavaScript can be done using:
- Console.log: Print messages to the browser console.
- Debugger statement: Pause execution and inspect variables in the browser’s developer tools.
- Browser Extensions: Use tools like Chrome Developer Tools or Firefox Developer Edition for more advanced debugging.
10. What are some best practices for writing JavaScript code?
To write efficient and maintainable JavaScript code:
- Use meaningful variable and function names.
- Follow consistent coding conventions and indentation.
- Always handle errors and exceptions properly.
- Optimize performance by minimizing DOM manipulation and using efficient algorithms.
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