The Top 25 JavaScript Interview Questions for Engineering Leaders

This guide will explore 25 essential JavaScript interview questions that cover a range of topics from basic to advanced concepts. Each question will include an explanation of why it's important, a sample answer, and when applicable, a code example. This comprehensive approach helps hiring managers assess the depth of a candidate's knowledge and their ability to apply JavaScript in practical scenarios ensuring that they hire Javascript developers that are the right fit.

1. What are the different data types present in JavaScript?

Explanation: Understanding data types is fundamental for any JavaScript programmer as they impact how values can be manipulated in the code.

Sample Answer: JavaScript has two types of data types: primitive (undefined, null, booleans, strings, and numbers, BigInt, Symbol) and non-primitive (objects including arrays and functions).

Code Example:

let string = "Hello, world!";
let number = 42;
let bigInt = 1234567890123456789012345678901234567890n;
let boolean = true;
let symbol = Symbol('a');
let object = {name: "John", age: 30};
let array = [1, 2, 3];
let func = function() { return "hello"; };

2. Explain hoisting in JavaScript

Explanation: Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope before code execution.

Sample Answer: In JavaScript, all variable and function declarations are hoisted to the top of their scope regardless of whether their scope is global or local. However, only the declarations are hoisted, not initializations.

Code Example:

console.log(num); // Outputs 'undefined', not the value
var num; // Declaration is hoisted
num = 6; // Initialization is not hoisted

3. What is the difference between == and === in JavaScript?

Explanation: This question tests the candidate's understanding of type coercion and equality checks in JavaScript.

Sample Answer: == is the loose equality operator that converts the operands to the same type before making the comparison. === is the strict equality operator that does not perform type conversion; it compares both type and value.

Code Example:

console.log(1 == "1"); // true, type coercion occurs
console.log(1 === "1"); // false, different types

4. Explain closures in JavaScript

Explanation: Closures are a fundamental concept in JavaScript that allows a function to access variables from an enclosing scope, even after the outer function has closed.

Sample Answer: A closure is a function bundled together with references to its surrounding state. It gives you access to an outer function’s scope from an inner function.

Code Example:

function makeAdder(x) {
 return function(y) {
 return x + y;
 };
}
var add5 = makeAdder(5);
console.log(add5(2)); // 7

5. What are JavaScript Promises and how do they work?

Explanation: Understanding promises is crucial for handling asynchronous operations in JavaScript.

Sample Answer: A Promise is an object representing the eventual completion or failure of an asynchronous operation. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason.

Code Example:

let promise = new Promise(function(resolve, reject) {
 setTimeout(() => resolve("done"), 1000);
});

promise.then(
 result => console.log(result), // 'done' after 1 second
 error => console.log(error) // does not run
);

6. Explain the event loop in JavaScript

Explanation: The event loop is a core concept of the JavaScript runtime used to handle asynchronous operations.

Sample Answer: JavaScript has a single-threaded, non-blocking event loop architecture. The event loop continuously takes events from the event queue and processes them if the call stack is empty.

Code Example:

console.log('First');
setTimeout(function() {
 console.log('Second');
}, 0);
console.log('Third');
// Output: First, Third, Second

7. What is the use of arrow functions?

Explanation: Arrow functions are a concise syntax for writing function expressions in JavaScript and they handle this differently.

Sample Answer: Arrow functions provide a shorter syntax compared to function expressions and do not have their own this, arguments, super, or new.target. They are best suited for non-method functions.

Code Example:

const add = (a, b) => a + b;
console.log(add(2, 3)); // 5

8. Can you describe the Prototype Chain in JavaScript?

Explanation: Prototype chain is a fundamental concept that JavaScript uses for object inheritance.

Sample Answer: Every JavaScript object has a prototype object from which it inherits methods and properties. A series of objects linked together via prototypes is known as the prototype chain.

Code Example:

function Person(name) {
 this.name = name;
}
Person.prototype.getName = function() {
 return this.name;
};
let person1 = new Person("Alice");
console.log(person1.getName()); // Outputs 'Alice'
console.log(person1.hasOwnProperty('getName')); // false
console.log(person1.__proto__.hasOwnProperty('getName')); // true

9. How do you create private variables in JavaScript?

Explanation: Encapsulation is an important aspect of robust JavaScript application design, and private variables are a way to achieve this.

Sample Answer: Private variables in JavaScript can be created using closures or, more recently, with the # syntax in classes.

Code Example:

// Using closures
function Counter() {
 let count = 0;
 this.incrementCounter = function() {
 count++;
 console.log(count);
 };
 this.decrementCounter = function() {
 count--;
 console.log(count);
 };
}
let counter = new Counter();
counter.incrementCounter(); // 1
counter.decrementCounter(); // 0
// Using class fields (ES2022+)
class CounterClass {
 #count = 0;
 increment() {
 this.#count++;
 console.log(this.#count);
 }
 decrement() {
 this.#count--;
 console.log(this.#count);
 }
}
let myCounter = new CounterClass();
myCounter.increment(); // 1
myCounter.decrement(); // 0

10. What is the nullish coalescing operator in JavaScript and how does it work?

Explanation: The nullish coalescing operator is a logical operator that provides a short syntax for default value assignments.

Sample Answer: The nullish coalescing operator (??) returns the right-hand operand when the left-hand operand is null or undefined, otherwise it returns the left-hand operand.

Code Example:

let age = null;
console.log(age ?? 25); // 25
age = 22;
console.log(age ?? 25); // 22

11. Explain the async/await syntax.

Explanation: The async/await syntax simplifies the process of working with promises, making asynchronous code easier to write and to read.

Sample Answer: async makes a function return a Promise, while await makes a function wait for a Promise. They help write asynchronous code that looks synchronous.

Code Example:

async function getApiData() {
 let response = await fetch('https://api.example.com/data');
 let data = await response.json();
 console.log(data);
}
getApiData();

12. Describe the difference between var, let, and const

Explanation: Differentiating these variable declarations is crucial for understanding scope, hoisting, and reassignment in JavaScript.

Sample Answer: var is function-scoped and can be redeclared and updated. let and const are block-scoped, making them more suitable for modern JavaScript development. let can be updated but not redeclared, while const cannot be updated or redeclared.

Code Example:

var x = 10;
{
 var x = 2; // Same variable!
 let y = 3; // Different variable
 const z = 4; // Different constant
}
console.log(x); // 2
// console.log(y); // ReferenceError: y is not defined
// console.log(z); // ReferenceError: z is not defined

13. How can you prevent a JavaScript object from being modified?

Explanation: Ensuring object immutability can be important in many development scenarios, particularly when working with constant configurations or state management.

Sample Answer: JavaScript provides several methods to prevent an object from being modified: Object.freeze(), Object.seal(), and using const to prevent reassignment of object references.

Code Example:

let obj = {name: "John"};
Object.freeze(obj);
obj.name = "Jane"; // No effect because the object is frozen
console.log(obj.name); // John

let obj2 = {name: "John"};
Object.seal(obj2);
obj2.name = "Jane"; // You can update properties
delete obj2.name; // But you cannot delete properties
console.log(obj2.name); // Jane

14. What is the Map object and how is it different from a JavaScript object?

Explanation: Map is a built-in object that provides the functionality of a dictionary, with some significant differences from standard JavaScript objects.

Sample Answer: Map is a collection of keyed data items, just like an Object. However, unlike Objects, Maps retain the order of their elements and can have keys of any datatype.

Code Example:

let map = new Map();
map.set('a', 1);
map.set(1, 'b');
map.set([1, 2, 3], 'arrayKey');
console.log(map.get('a')); // 1
console.log(map.get(1)); // 'b'
console.log(map.get([1, 2, 3])); // undefined, different reference

15. Explain how this keyword works in JavaScript

Explanation: The this keyword is a core concept in JavaScript, essential for understanding how execution contexts work within functions.

Sample Answer: In JavaScript, this refers to the object that the function is a part of. However, the value of this depends on how the function is called. It can change based on the execution context unless bound explicitly with methods like .bind().

Code Example:

function show() {
 console.log(this);
}
show(); // global object (in non-strict mode), undefined in strict mode

let obj = {
 show: function() {
 console.log(this);
 }
};
obj.show(); // the obj itself

16. What are JavaScript Modules?

Explanation: Modules are an essential feature for maintaining clean code in larger JavaScript projects by allowing you to divide and organize your code among multiple files.

Sample Answer: JavaScript modules are files containing JavaScript code that can export functions, objects, or primitive values. These can then be imported by other modules.

Code Example:

// math.js
export const add = (x, y) => x + y;
export const subtract = (x, y) => x - y;

// app.js
import { add, subtract } from './math.js';
console.log(add(5, 3)); // 8
console.log(subtract(5, 3)); // 2

17. How do you handle exceptions in JavaScript?

Explanation: Exception handling is crucial for managing errors gracefully in JavaScript applications to maintain a robust and fault-tolerant system.

Sample Answer: JavaScript uses try, catch, finally blocks to handle exceptions. Code that may throw an exception is put inside a try block. catch is used to catch and handle the exception if one occurs. finally executes after the try and catch blocks irrespective of whether an exception was caught.

Code Example:

try {
 throw new Error('Oops!');
} catch (e) {
 console.log(e.name + ": " + e.message);
} finally {
 console.log('This will run no matter what');
}
// Output: Error: Oops!
// This will run no matter what

18. What are template literals in JavaScript?

Explanation: Template literals are a relatively new feature that provide a way to create multi-line strings and perform string interpolation among other features.

Sample Answer: Template literals are enclosed by backticks (`) instead of single or double quotes. They can contain placeholders, which are indicated by ${expression}, where the expression can be any JavaScript expression.

Code Example:

let name = "World";
let message = `Hello, ${name}!`;
console.log(message); // Hello, World!

19. What are service workers and how do they work?

Explanation: Service workers are an important feature for creating robust, high-performance web applications, particularly for offline use cases.

Sample Answer: A service worker is a script that your browser runs in the background, separate from a web page, enabling features that don't need a web page or user interaction. They are predominantly used for background sync, caching, and listening to network requests.

Code Example:

// In your main JavaScript file
if ('serviceWorker' in navigator) {
 navigator.serviceWorker.register('/sw.js').then(function(registration) {
 console.log('Service Worker registered with scope:', registration.scope);
 }).catch(function(error) {
 console.log('Service Worker registration failed:', error);
 });
}

// In sw.js (service worker file)
self.addEventListener('install', function(event) {
 event.waitUntil(
 caches.open('v1').then(function(cache) {
 return cache.addAll([
 '/index.html',
 '/style.css',
 '/app.js'
 ]);
 })
 );
});

20. Explain the concept of "destructuring" in JavaScript

Explanation: Destructuring is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.

Sample Answer: Destructuring allows you to bind a set of variables to a corresponding set of values anywhere that you can bind variables, including in function parameter positions, assignments, and variable declarations.

Code Example:

let a, b, rest;
[a, b] = [10, 20];
console.log(a); // 10
console.log(b); // 20

({a, b} = {a: 10, b: 20});
console.log(a); // 10
console.log(b); // 20

// With rest parameter
[a, b, ...rest] = [10, 20, 30, 40, 50];
console.log(rest); // [30, 40, 50]

21. What are higher-order functions in JavaScript?

Explanation: Higher-order functions are a concept borrowed from functional programming that involve functions taking other functions as arguments or returning them as output.

Sample Answer: Higher-order functions are functions that operate on other functions by taking them as arguments or by returning them. The classic examples are array methods like map, filter, and reduce.

Code Example:

const numbers = [1, 2, 3, 4, 5];

const doubled = numbers.map(number => number * 2);
console.log(doubled); // [2, 4, 6, 8, 10]

const even = numbers.filter(number => number % 2 === 0);
console.log(even); // [2, 4]

const sum = numbers.reduce((acc, number) => acc + number, 0);
console.log(sum); // 15

22. How do you implement inheritance in JavaScript?

Explanation: Understanding inheritance is key to leveraging JavaScript's prototype-based object model effectively.

Sample Answer: JavaScript uses prototypical inheritance. Each object has a private property which holds a link to another object called its prototype. This prototype object has a prototype of its own, and so on until an object is reached with null as its prototype.

Code Example:

function Animal(name) {
 this.name = name;
}
Animal.prototype.speak = function () {
 console.log(`${this.name} makes a noise.`);
}

function Dog(name, breed) {
 Animal.call(this, name); // call super constructor.
 this.breed = breed;
}

// Inheriting Animal's methods
Dog.prototype = Object.create(Animal.prototype);

Dog.prototype.speak = function () {
 console.log(`${this.name} barks.`);
};

let d = new Dog('Mitzie', 'Shih Tzu');
d.speak(); // Mitzie barks.

23. What is the purpose of the Array.map() method?

Explanation: Understanding array operations like map is crucial for working with data collections in JavaScript.

Sample Answer: The map() method creates a new array populated with the results of calling a provided function on every element in the calling array. This is particularly useful for transforming data.

Code Example:

const array = [1, 4, 9, 16];
const map1 = array.map(x => x * 2);
console.log(map1);
// expected output: Array [2, 8, 18, 32]

24. What are the benefits of using async/await over Promises?

Explanation: Comparing async/await and Promises helps to understand asynchronous programming patterns in JavaScript.

Sample Answer: async/await makes the syntax and flow of your asynchronous code more readable and concise. It reduces the complexity of chaining promises and handling errors in .catch() or .then() methods.

Code Example:

// Using Promises
function getJSON(url) {
 return fetch(url)
 .then(response => response.json())
 .catch(error => console.log('failed to fetch data:', error));
}

// Using async/await
async function getJSONAsync(url) {
 try {
 let response = await fetch(url);
 let data = await response.json();
 return data;
 } catch (error) {
 console.log('failed to fetch data:', error);
 }
}

25. What are mixins and how can they be useful in JavaScript programming?

Explanation: Mixins are a tool for reusing code in multiple class hierarchies.

Sample Answer: A mixin is a class that offers certain functionality to be inherited by a subclass, but is not intended to stand alone. JavaScript allows mixin functionalities by copying methods from a source object to a target object.

Code Example:

let sayHiMixin = {
 sayHi() {
 console.log(`Hello ${this.name}`);
 },
 sayBye() {
 console.log(`Bye ${this.name}`);
 }
};

class User {
 constructor(name) {
 this.name = name;
 }
}

// Copy the methods
Object.assign(User.prototype, sayHiMixin);

// Now User can say hi
let user = new User("Dmitry");
user.sayHi(); // Hello Dmitry

This set of 25 JavaScript interview questions

This set of 25 JavaScript interview questions spans from basic to advanced topics, providing a broad spectrum of what a candidate might face in a technical interview. Understanding these concepts and being able to discuss them in detail will not only help in identifying qualified candidates but also in gauging their depth of understanding and practical skills in JavaScript programming. For even more hiring resources, see our guide on how to interview a javascript developer.