30 Top JavaScript Interview Questions for Freshers [2023 UPDATED]
Preparing for your first JavaScript developer role? Master these interview questions
As a fresher, one of the most daunting tasks you’ll face is preparing for your first job interview. Especially when it comes to the field of JavaScript, which is a versatile and constantly evolving programming language. To help you ace your next interview and land your dream job, I have compiled a list of the most commonly asked JavaScript interview questions.
These questions cover a wide range of topics, including basic JavaScript syntax, object-oriented programming concepts, data structures, and common libraries and frameworks. By familiarizing yourself with the answers to these questions, you’ll be well-prepared to demonstrate your knowledge and skills to potential employers.
In this blog post, I will take you through the most important JavaScript interview questions that are frequently asked by interviewers for freshers. I will also provide detailed explanations and examples for each question to help you understand the concepts better and to be able to answer confidently.
So, whether you’re a beginner just starting to learn JavaScript or an experienced developer looking to brush up on your skills, this guide is for you. By the end of this blog post, you’ll have a solid understanding of the key concepts and be ready to tackle any question that comes your way in a JavaScript interview.
What is JavaScript and how is it different from Java?
JavaScript is a scripting language that is primarily used to create dynamic and interactive web pages, while Java is a programming language that is primarily used to create standalone applications.
What are the data types in JavaScript?
JavaScript has six data types: Number, String, Boolean, Symbol, Object, and Undefined.
Explain hoisting in JavaScript.
Hoisting is a behavior in JavaScript where variables and function declarations are moved to the top of their scope. This means that a variable can be used before it is declared.
What is the difference between let and var in JavaScript?
Both let and var are used to declare variables in JavaScript, but there are a few key differences. Variables declared with var are function-scoped, while variables declared with let are block-scoped. This means that a variable declared with var can be accessed throughout the entire function, while a variable declared with let can only be accessed within the block it was declared in.
What is closure in JavaScript?
A closure is a function that has access to variables in its parent scope, even after the parent function has completed execution. This allows for data privacy and encapsulation in JavaScript.
How do you declare a variable in JavaScript?
Variables in JavaScript can be declared using the keywords var, let, or const. Example: let x = 5;
What is an event loop in JavaScript?
An event loop is a mechanism that allows JavaScript to execute code in a non-blocking way. It works by allowing the browser to continue executing other code while it waits for an event, such as a button click or a network request, to occur.
What is a callback function in JavaScript?
A callback function is a function that is passed as an argument to another function and is executed after the parent function has completed. This allows for a non-blocking execution of code and allows for the separation of concerns in the code. Example: setTimeout(function(){console.log("Hello World")},1000);
What is the difference between == and === in JavaScript?
The double equals == compares values for equality, while the triple equals === compares both values and types for equality.
What is an object in JavaScript?
An object in JavaScript is a collection of key-value pairs, similar to a dictionary or a hash table in other languages. Objects can be created using the object literal notation {key: value} or the object constructor new Object().
What is a prototype in JavaScript?
A prototype in JavaScript is an object that acts as a template for other objects. When an object is created, it inherits the properties and methods of its prototype. Objects can also have their own properties and methods added to them.
Example:
function Person(name, age) {
this.name = name;
this.age = age;
}
Person.prototype.sayName = function() {
console.log(this.name);
}
let person1 = new Person("John", 25);
console.log(person1.name); // Output: John
console.log(person1.age); // Output: 25
person1.sayName(); // Output: JohnWhat is an async function in JavaScript?
An async function is a function that returns a promise and can be paused using the await keyword. This allows for a more readable and organized way of handling asynchronous code.
Example:
async function getData() {
let response = await fetch('https://jsonplaceholder.typicode.com/posts/1');
let data = await response.json();
console.log(data);
}
getData()What is a promise in JavaScript?
A promise in JavaScript is an object that represents the eventual completion or failure of an asynchronous operation. Promises have a .then() method that is called when the promise is fulfilled, and a .catch() method that is called when the promise is rejected.
Example:
let promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Data Loaded");
}, 3000);
});
promise.then((data) => {
console.log(data);
});What is the difference between null and undefined in JavaScript?
Both null and undefined represent the absence of a value, but they are used in different contexts. undefined is used when a variable is declared but has not been assigned a value yet, while null is used to explicitly set a variable to have no value.
What is a module in JavaScript?
A module in JavaScript is a way to organize and reuse code by encapsulating it into a single unit. Modules can be imported and exported using the import and export keywords.
//file: myModule.js
export const myVariable = "Hello";
export function myFunction() {
console.log("World");
}
//file: main.js
import { myVariable, myFunction } from './myModule';
console.log(myVariable); // Output: "Hello"
myFunction(); // Output: "World"What is a spread operator in JavaScript?
The spread operator ... is used to expand an iterable (e.g. an array or a string) into individual elements or properties.
Example:
let array1 = [1, 2, 3];
let array2 = [...array1, 4, 5];
console.log(array2); // Output: [1, 2, 3, 4, 5]What is a map function in JavaScript?
The map function is a higher-order function that is used to transform an array by applying a callback function to each element of the array. It returns a new array with the transformed elements.
Example:
let numbers = [1, 2, 3, 4, 5];
let doubledNumbers = numbers.map((number) => {
return number * 2;
});
console.log(doubledNumbers); // Output: [2, 4, 6, 8, 10]What is a filter function in JavaScript?
The filter function is a higher-order function that is used to filter an array based on a certain condition. It returns a new array containing only the elements that pass the condition.
Example:
let numbers = [1, 2, 3, 4, 5];
let evenNumbers = numbers.filter((number) => {
return number % 2 === 0;
});
console.log(evenNumbers); // Output: [2, 4]What is a reduce function in JavaScript?
The reduce function is a higher-order function that is used to reduce an array to a single value by applying a callback function to each element of the array. It takes an accumulator and a current value as arguments and returns a new accumulator value.
Example:
let numbers = [1, 2, 3, 4, 5];
let sum = numbers.reduce((accumulator, currentValue) => {
return accumulator + currentValue;
}, 0);
console.log(sum); // Output: 15What is a generator function in JavaScript?
A generator function is a special type of function that can be paused and resumed using the yield and next() keywords. It allows for a more efficient way of handling infinite data streams or complex data flows.
Example:
function* generatorFunction() {
yield 1;
yield 2;
yield 3;
}
let generator = generatorFunction();
console.log(generator.next().value); // Output: 1
console.log(generator.next().value); // Output: 2
console.log(generator.next().value); // Output: 3What is a class in JavaScript?
A class in JavaScript is a blueprint for creating objects (instances) that share the same properties and methods. It is a more elegant way of creating objects and encapsulating related data and behavior.
Example:
class Person {
constructor(name, age) {
this.name = name;
this.age = age;
}
sayName() {
console.log(this.name);
}
}
let person1 = new Person("John", 25);
console.log(person1.name); // Output: John
console.log(person1.age); // Output: 25
person1.sayName(); // Output: JohnWhat is the difference between a class and a constructor function in JavaScript?
Both classes and constructor functions are used to create objects, but classes have a more elegant syntax and are more similar to object-oriented languages like Java and C#. Constructor functions are more similar to the traditional way of creating objects in JavaScript.
What is a try-catch block in JavaScript?
A try-catch block is used to handle errors and exceptions in JavaScript. The code inside the try block is executed and if an error occurs, the code inside the catch block is executed.
Example:
try {
let a = 5;
let b = 0;
let c = a / b;
console.log(c);
} catch (error) {
console.log("Error:", error.message);
}What is a decorator in JavaScript?
A decorator is a design pattern that allows for the modification of a class or method at runtime. It is a way to add additional functionality to existing code without modifying the original code.
What is a named function in JavaScript?
A named function is a function that is given a name when it is defined, and can be called by that name later on in the code.
function add(a, b) {
return a + b;
}
console.log(add(1, 2)); // Output: 3What is an anonymous function in JavaScript?
An anonymous function is a function that does not have a name, it is defined and called on the fly.
let add = function (a, b) {
return a + b;
};
console.log(add(1, 2)); // Output: 3What is a DOM in JavaScript?
The Document Object Model (DOM) is an object-oriented representation of an HTML or XML document that can be manipulated using JavaScript. It allows for the manipulation of the structure, content, and styles of a webpage.
What is a jQuery?
jQuery is a JavaScript library that simplifies the manipulation of the DOM, handling of events, and making of AJAX requests. It is widely used to make web development more efficient and consistent across different browsers.
What is a single-page application?
Single-page application (SPA) is a web application or website that loads a single HTML page and dynamically updates the content as the user interacts with the application.
I hope that the questions and answers provided in this blog post have helped you to better understand the key concepts of JavaScript and have given you the confidence to tackle any question that comes your way in an interview.
Remember that practice makes perfect and the more you familiarize yourself with the questions and answers, the more comfortable and confident you will feel during your interview.
Best of luck with your next JavaScript interview and your future endeavors as a JavaScript developer. Keep learning and keep growing.
