JavaScript remains one of the most sought programming languages, and it is still in high demand in the tech industry to this very day. For those developers who are on the advanced level, the interview questions can either be on the nitty-gritty of the language, various best practices and advanced topics. Thus, we offer a list of the most popular JavaScript interview questions for experienced developers along with their answers and explanations, and this time we have chosen the top 25 questions.
Table of Contents
1. What is the difference between let, const, and var?
varIt is function-scoped, can be re-declared and updated.let: It is block-scoped, it cannot be re-declared within the same block, but can be updated.const: Again, it is block-scoped, cannot be re-declared or updated (immutable binding).
2. Explain the concept of closures in JavaScript.
Closure is a function that remembers its outer variables and can access them. In JavaScript, a closure is created every time a function is created, at function creation time.
function outerFunction() {
let outerVariable = 'I am from outer scope';
function innerFunction() {
console.log(outerVariable);
}
return innerFunction;
}
const myClosure = outerFunction();
myClosure(); // Outputs: I am from outer scope 3. What is the event loop and how does it work?
Event loop is a mechanism that allows JavaScript to perform non-blocking operations by using callbacks and promises. It continuously checks the call stack and the callback queue. If the call stack is empty, it takes the first event from the queue and pushes it to the call stack.
4. Explain the concept of promises and async/await in JavaScript.
Promises represent a value that may be available now, in the future, or never. Async functions are a higher-level abstraction over promises, making asynchronous code look more like synchronous/procedural code.
async function fetchData() {
try {
let response = await fetch('https://api.example.com/data');
let data = await response.json();
console.log(data);
} catch (error) {
console.error(error);
}
} 5. What is the purpose of JavaScript modules and how do you use them?
Modules allow you to split your code into separate files for better maintainability and reusability. In ES6, you can use export and import to share code between files.
// module.js
export const myFunction = () => {
console.log('Hello from a module!');
}
// main.js
import { myFunction } from './module.js';
myFunction(); // Outputs: Hello from a module! 6. Describe the differences
== is the equality operator and performs type coercion if the types differ. === is the strict equality operator and checks for both value and type equality.
console.log(5 == '5'); // true
console.log(5 === '5'); // false 7. How does prototypal inheritance work in JavaScript?
JavaScript uses prototypal inheritance, meaning objects can inherit properties from other objects. Each object has a private property called [[Prototype]] which can be set to another object, allowing the object to inherit properties and methods from its prototype.
const parent = {
greet: function() {
console.log('Hello from parent');
}
};
const child = Object.create(parent);
child.greet(); // Outputs: Hello from parent 8. What are higher-order functions in JavaScript?
Higher-order functions are functions that can take other functions as arguments, return a function as a result, or both. They are a key feature of functional programming.
function higherOrderFunction(callback) {
return function() {
callback();
}
}
const sayHello = () => console.log('Hello!');
const newFunction = higherOrderFunction(sayHello);
newFunction(); // Outputs: Hello! 9. Explain the concept of currying in JavaScript.
Currying is a technique of transforming a function that takes multiple arguments into a sequence of functions that each take a single argument.
function curry(f) {
return function(a) {
return function(b) {
return f(a, b);
};
};
}
const add = (a, b) => a + b;
const curriedAdd = curry(add);
console.log(curriedAdd(1)(2)); // Outputs: 3 10. What are JavaScript generators and how do they work?
Generators are functions that can be paused and resumed. They use the function* syntax and the yield keyword.
function* generatorFunction() {
yield 'First output';
yield 'Second output';
return 'Final output';
}
const generator = generatorFunction();
console.log(generator.next().value); // Outputs: First output
console.log(generator.next().value); // Outputs: Second output
console.log(generator.next().value); // Outputs: Final output 11. What is the this keyword and how is it determined?
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. - Alone,
thisrefers to the global object. - In a function,
thisrefers to the global object (in non-strict mode). - In an event,
thisrefers to the element that received the event. - In arrow functions,
thisretains the value of the enclosing lexical context.
12. Explain the differences between call, apply, and bind.
call: Invokes a function with a giventhisvalue and arguments provided individually.apply: Invokes a function with a giventhisvalue and arguments provided as an array.bind: Creates a new function that, when called, has itsthiskeyword set to the provided value, with a given sequence of arguments preceding any provided when the new function is called.
function greet(greeting, punctuation) {
console.log(greeting + ', ' + this.name + punctuation);
}
const person = { name: 'John' };
greet.call(person, 'Hello', '!'); // Outputs: Hello, John!
greet.apply(person, ['Hi', '.']); // Outputs: Hi, John.
const boundGreet = greet.bind(person, 'Hey');
boundGreet('!!!'); // Outputs: Hey, John!!! 13. What are JavaScript design patterns and why are they useful?
Design patterns are proven solutions to common problems in software design. They are templates for solving issues and can improve code readability, maintainability, and scalability. Examples include the Singleton, Observer, and Module patterns.
14. Explain the concept of immutability in JavaScript and how you can achieve it.
Immutability means that an object’s state cannot be modified after it is created. In JavaScript, you can achieve immutability by using const, Object.freeze(), and by not modifying objects directly.
const obj = Object.freeze({ name: 'John' });
obj.name = 'Doe'; // This will not change the name property
console.log(obj.name); // Outputs: John 15. What is memoization and how can you implement it in JavaScript?
Memoization is an optimization technique that stores the results of expensive function calls and returns the cached result when the same inputs occur again.
function memoize(fn) {
const cache = new Map();
return function(...args) {
const key = JSON.stringify(args);
if (cache.has(key)) {
return cache.get(key);
}
const result = fn(...args);
cache.set(key, result);
return result;
};
}
const factorial = memoize(function(n) {
if (n <= 1) return 1;
return n * factorial(n - 1);
});
console.log(factorial(5)); // Outputs: 120
console.log(factorial(5)); // Outputs: 120 (from cache) 16. What are service workers and how do they work?
Service workers are scripts that run in the background, separate from the web page, enabling features like push notifications and background sync. They provide the foundation for progressive web apps (PWAs).
17. Explain the concept of debouncing and throttling in JavaScript.
Debouncing and throttling are techniques to control how often a function should be executed.
- Debouncing: Ensures a function is only called after a certain period has elapsed since the last time it was invoked.
function debounce(func, wait) {
let timeout;
return function(...args) {
clearTimeout(timeout);
timeout = setTimeout(() => func.apply(this, args), wait);
};
} - Throttling: Ensures a function is called at most once in a specified period.
function throttle(func, limit) {
let inThrottle;
return function(...args) {
if (!inThrottle) {
func.apply(this, args);
inThrottle = true;
setTimeout(() => (inThrottle = false), limit);
}
};
} 18. What is the difference between synchronous and asynchronous code in JavaScript?
- Synchronous: Code is executed sequentially, one line at a time.
- Asynchronous: Code can run in the background, allowing other operations to continue without waiting for the previous one to complete.
19. Explain the concept of the spread operator and its uses.
The spread operator (...) allows an iterable (like an array or string) to be expanded in places where zero or more arguments (for function calls) or elements (for array literals) are expected.
const arr = [1, 2, 3];
const newArr = [...arr, 4, 5, 6];
console.log(newArr); // Outputs: [1, 2, 3, 4, 5, 6] 20. What is the difference between null and undefined in JavaScript?
undefined: A variable has been declared but has not yet been assigned a value.null: An assignment value that represents “no value” or “no object”.
21. How do you handle errors in JavaScript?
Errors can be handled using try...catch blocks.
try {
// Code that may throw an error
throw new Error('An error occurred');
} catch (error) {
// Handle the error
console.error(error.message);
} finally {
// Code that will run regardless of error
console.log('This will always run');
} 22. Explain the concept of hoisting in JavaScript.
Hoisting is JavaScript’s default behavior of moving declarations to the top of the scope. Only the declarations are hoisted, not the initializations.
console.log(hoistedVar); // Outputs: undefined
var hoistedVar = 'This is hoisted'; 23. What are arrow functions and how do they differ from regular functions?
Arrow functions provide a shorter syntax and do not have their own this, arguments, super, or new.target. They are always anonymous.
const add = (a, b) => a + b;
console.log(add(2, 3)); // Outputs: 5 24. Explain the concept of event delegation in JavaScript.
Event delegation is a technique involving event listeners to a parent element instead of multiple child elements. The event listener analyzes bubbled events to find a match on child elements.
document.getElementById('parent').addEventListener('click', function(event) {
if (event.target && event.target.matches('button.className')) {
console.log('Button clicked!');
}
}); 25. What are the different ways to create objects in JavaScript?
Objects can be created using:
- Object literals
- The
newkeyword with a constructor function - The
Object.create()method - ES6 classes
const obj1 = { name: 'John' };
const obj2 = new Object();
obj2.name = 'Doe';
function Person(name) {
this.name = name;
}
const obj3 = new Person('Jane');
const obj4 = Object.create(obj1);
class Animal {
constructor(name) {
this.name = name;
}
}
const obj5 = new Animal('Dog'); Conclusion
Getting to master the body of knowledge related to the interview questions focused on JavaScript would entail a lot which comprises internalization of the principles that form the foundation of the javascript programming language as well as practical exposure to the high level features of the language. if mastering these top 25 JavaScript interview questions and answers for experienced developers is meant for sharpening your skills, enabling you even to pass any technical interview you may face, then it is a very efficient method of preparing yourself for any technical interview.
To summarize, be aware that the interview question is not only to answer correctly, the more important aspect is to comprehend the main idea as well as to express the thactual logic internally and exhaustively, and externally and concisely. Solve these questions, implement projects, and keep yourself informed regarding new features of javascript so that you do well in your future.
All the best as you prepare for the interview, and may your next position bring you closer to your dream of becoming a great Java Script developer!