Top 10 Important JavaScript Interview Questions You Need to Know

JavaScript Interview Questions

Looking for JavaScript interview questions? Here are 10  essential ones that you are likely to be quizzed on.

Alongside HTML and CSS, JavaScript is one of the core technologies of the World Wide Web. JavaScript enables interactive web pages and is an essential part of web applications. The vast majority of websites use it, and major web browsers have a dedicated JavaScript engine to execute it. Source: Wikipedia

1. What is JavaScript?

JavaScript is a scripting language primarily designed for creating web pages and making web applications interactive.

2. How does JavaScript work?

Interview questions for JavaScript without one on how JS works? Not possible. Every web browser works using three prime components. The first one is the DOM (Document Object Model) interpreter. This takes your HTML document, converts it, and displays it in the browser. The second component is a CSS interpreter which styles the page and gives the visual effects. The last component of a browser is the JS engine.

  1. The browser loads the HTML file or JS file.
  2. JavaScript is an interpreted language and does not require compilation.
  3. The Browser (using the JavaScript engine) executes the file line by line and waits for events (clicks, mouse hover, etc.) to happen.

3. What are some of the features of JavaScript?

Some prominent features of JavaScript are:

  • JavaScript is a lightweight programming language with interpreted functionality.
  • It is open-source and cross-platform.
  • JavaScript is integrated within HTML and Java.
  • The language is designed to create network-centric applications.

4. What are the different data types in JavaScript?

The different data types in JavaScript are:

  • Strings
  • Functions
  • Boolean
  • Object
  • Number
  • Undefined

5. What are the different errors in JavaScript?

In general, there are 3 types of errors in JavaScript (or JS):

  • Runtime error: This error occurs on the misuse of HTML commands.
  • Load tie error: A syntax error that is generated dynamically.
  • Logical error: This error occurs when there is some issue with a function’s logic.

Must Read: 10 Effective and Essential Angular JavaScript Interview Questions

6. Is JavaScript case-sensitive or case-insensitive?

JavaScript is a case-sensitive programming language. In JS, we use different types of variables, functions, and other identities that need to be consistent throughout.

7. What are some advantages and disadvantages of JavaScript?

Three advantages of JavaScript are:

  • Rich user interface.
  • Increased interactivity (when a mouse hovers on elements like buttons or keyboard accessibility).
  • Allows both front-end and back-end development.

Three disadvantages of JavaScript are:

  1. Lacks multi-threading activities.
  2. Security is an issue since the user can view the JavaScript code.
  3. Different browsers treat JavaScript differently. Hence, the behavior may change depending on the browser.

8. How many types of objects are there in JavaScript?

There are 2 types of objects in JS:

  • Date Object: This object is built within the JS programming. language It is created with the use of a new date and can be operated using the methods once it is created. This includes the year, month, day, hour, minutes, seconds, and even milliseconds of the date object. These values are set with the help of local standards of universal time.
  • Number Object: It is represented by integers and fractions. The literals of the numbers get converted to number class automatically.

9. What is closure in JavaScript?

A closure gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created, at function creation time. There are three scopes:

  • Variables declared in their own scope
  • Variables declared in a parent function scope
  • Variables declared in the global namespace

var globalVar = “abc”;

// Parent self invoking function
(function outerFunction (outerArg) { // begin of scope
// Variable declared in outerFunction function scope
var outerFuncVar = ‘x’;
// Closure self-invoking function
(function innerFunction (innerArg) { // begin of scope
// variable declared in innerFunction function scope
var innerFuncVar = “y”;
“outerArg = ” + outerArg + “n” +
“outerFuncVar = ” + outerFuncVar + “n” +
“innerArg = ” + innerArg + “n” +
“innerFuncVar = ” + innerFuncVar + “n” +
“globalVar = ” + globalVar);

}// end of scope innerFunction)(5); // Pass 5 as parameter
}// end of scope outerFunction )(7); // Pass 7 as parameter

innerFunction is a closure that is defined inside the outerFunction and has access to all variables declared and defined in the outer function scope. Moreover, the function defined inside another function as a closure will have access to variables declared in the global namespace.

outerArg = 7
outerFuncVar = x
innerArg = 5
innerFuncVar = y
globalVar = abc

10. How do you empty an array in JavaScript?

An array can be cleared in the following ways:

  • myArray = [ ]

The statement will set the variable myArray to a new empty array.

  • length = 0;

The statement clears the existing array by setting its length to 0. This approach is useful when you want to update all of the references pointing to the array.

  • splice(0, myArray.length);

This way of emptying the array will also update the references of the original array.

  • while(myArray.length){



This is another method of emptying the array.

If you think there are some important JavaScript interview questions that candidates need to know about, do comment.

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