JavaScript Common Interview Questions and Answers [PDF]

JavaScript Common Interview Questions and Answers

The giant tech firms such as Facebook, Google, PayPal, and others use JavaScript for building complex web applications and it is the most efficient and widely-used programming language for building software.

Due to its ever-growing usage and popularity in the tech world, many people are aspiring to build their careers in JavaScript. And, if you are among those who are planning to step into the world of JavaScript, then you need to be well prepared — technical interviews, commonly asked questions, projects, etc.

Keeping this in mind we have prepared a few JS interview questions along with the answers which will surely help you in cracking the technical round during the selection process.

We will also be providing a downloadable PDF at the end of the blog post.

JavaScript common interview questions

Let’s go through all the questions and their answers one by one:

1. What is the difference between JavaScript & Java?

Some of the main differences between JavaScript and Java are:

JavaScript	Java
JavaScript is a prototype-based OOP scripting language	Java is a class-based OOP programming language
JavaScript doesn’t need to be compiled	Java needs to be compiled before execution
JavaScript code is used to run in browsers only, but with Node.js it can run on the server as well	Java applications can run on any virtual machine (JVM)
In JS, objects are prototype-based	In Java, the objects are class-based
JS doesn’t support multi-threading	Java supports multi-threading

2. Define JavaScript.

It is a lightweight, interpreted programming language with object-oriented abilities allowing programmers to build interactivity into otherwise static HTML pages.

3. What are the data types supported by JavaScript?

Following are the data types that are supported by JS:

Object
Undefined
Null
Boolean
String
Symbol
Number

4. JavaScript and an ASP script, which one is faster?

ASP is a server-side language and JavaScript is both server-side (Node.js) and client-side language. However, when it comes to speed, JavaScript is definitely faster because Node.js is a single-threaded event-driven asynchronous language.

Hence, JavaScript is faster.

5. Name the JavaScript developing company.

JavaScript was developed by Brendan Eich of Netscape in 1995.

6. What do you understand by undeclared and undefined variables?

Undeclared variables do not exist in a program and are not declared and a runtime error is encountered, in case a program tries to read the value of such variables.

However, undefined variables are the variables declared within the program but are not assigned any value as of now.

7. Define global variables.

Variables that are defined outside of functions are called global variables. These variables can be used by any function in the program since they have a global scope, which means all functions and scripts in the program can access it.

For example:

8. What is the difference between array splice and slice methods?

The array splice method returns the removed item by adding or removing items from an array, whereas the array slice method is used to return the selected items in an array as a new array object.

In other words, the splice method is used to insert or delete items to/from an array but the slice method is used to choose certain items/elements from an array.

For example:

9. What is the use of variable typing in JavaScript?

JavaScript allows you to reassign data of a different data type to the same variable, either accidentally or on purpose. This means JS is able to automatically adjust the data type.

For example:

10. What do you mean by NULL, in JavaScript?

NULL is used to define no object or no value, its syntax is null.

It implies a null string, no number, no value, no object, no valid Boolean, and no array object.

11. Distinguish between “==” and “===”.

In JavaScript, “==” it’s a coercion operator that only checks for equality in value.

While, “===” is a strict equality test and returns “true” in case, when both the variables are of the same type and contain the same value.

For example, in the case of null and undefined,

null === undefined   // false
null  == undefined   // true

12. What is Implicit Type Coercion in JavaScript? Give an example.

Type coercion refers to the implicit conversion of values from one data type to another (e.g. number to string or vice versa).

For example:

const first = '10';
const second = 20;

console.log(first + second); //returns 1020

In the example above, you can see how the second variable got converted to a string, and then the both first and second got concatenated.

13. What is passed by value and passed by reference?

In JavaScript, primitive data types (like string, number, boolean, undefined, and null) are passed by value whereas all the objects (non-primitive data types) are passed by reference.

14. What is a callback function?

If a function is passed into a different function as an argument, it’s called a callback function. These are of two types — synchronous and asynchronous callbacks.

A callback function runs only after another function has been executed and we need callback functions because JavaScript is an event-driven scripting language.