100 JavaScript Interview Questions and Answers in 2021

Ace your next programming interview

Photo by Maranda Vandergriff on Unsplash

Here is a comprehensive list of 100 programming interview questions and answers for you. In addition to interviews, these can be handy if you’re studying for a JavaScript examination.

Even if you are not participating in a programming interview or exam, this list is worthwhile—It goes through most of the important concepts of JavaScript.

Let’s get started.

1. What is JavaScript?

JavaScript is a client-side/server-side programming language. JavaScript can be inserted into HTML to make a web page interactive and enable user engagement.

2. What is hoisting in JavaScript?

Hoisting means all the declarations are moved to the top of the scope. This happens before the code is run.

For functions, this means you can call them from anywhere in the scope, even before they are defined.

For variables, hoisting is a bit different. It assigns undefined to them at the top of the scope.

For example, calling a variable before defining it:

console.log(dog);
var dog = "Spot";

Results in:

undefined

This may be surprising because you probably expected it to cause errors.

If you declare a function or variable, it is always moved on the top of the scope no matter where you declare it.

3. What does the isNan() function do?

You can use isNan() function to check if the type of a value is a number and it’s NaN.

(And yes, NaN is of type number even though it’s a “Not-a-Number” by name. This is because it’s a numeric type under the hood, even though it does not have a numeric value.)