10 Sample JavaScript Interview Questions and Answers

As a web developer, you know how to turn JavaScript code into responsive design and complex web functionality. But how do you prove to potential employers that you know your stuff? Start by reading through the sample JavaScript interview questions and answers below.

The perks of being a web developer include job security and excellent pay. According to the Bureau of Labor Statistics, we’ll likely see a 23% increase in new web developer jobs over the next decade. When you make a great first impression on a hiring manager, one of these jobs could be yours.

Study the sample JavaScript interview questions and answers outlined in this article. Then, take some time to script your own answers and practice them in a mock interview.

JavaScript Interview Questions and Answers

1. What Is JavaScript and How Does It Differ From Java?
2. What Are the Key JavaScript Data Types?
3. Describe What Hoisting Is in JavaScript.
4. What Are the Main Types of Variables in JavaScript?
5. What Are Methods in JavaScript and How Do They Work?
6. Tell Me About NaN in JavaScript.
7. What Is the Difference Between Passed By Value and Passed By Reference?
8. Describe How to Change the Style and Class of an Element.
9. What Is the “this” Keyword in JavaScript?
10. Which Types of Errors Might You Encounter in JavaScript and How Would You Address Them?

Question #1: What Is JavaScript and How Does It Differ from JAVA?

You’ve likely mentioned JavaScript on your web developer resume. Now, you’ll need to expand on that in your interview to show employers you have a basic understanding of what JavaScript is and what its essential functions are.

Here’s what you should include in your answer when responding to basic JavaScript interview questions:

• Provide a brief overview of what JavaScript is and why it’s used.
• Discuss the key similarities and differences between JavaScript and Java.

How You Could Answer

“JavaScript is a dynamic, text-based programming language that’s often used to develop websites, apps, games, and software. It allows developers to animate images, create image carousels, implement hover effects over buttons, play audio and video, add drop-down hamburger menus, and much more.

JavaScript is sometimes confused with Java, but they are not the same. Java is an object-oriented programming language, while JavaScript is an object-oriented programming script. Both can be used on the server side, but only JavaScript can be used on the client side. Additionally, Java must be compiled, while JavaScript only needs to be interpreted from the text.”

Question #2: What Are the Key JavaScript Data Types?

As a web, app, or software developer, you should understand JavaScript data types and be able to communicate what you know to a hiring manager.

Your answer should include:

• An overview of the most common data types in JavaScript.
• A brief description of each data type.

How You Could Answer

“JavaScript has five primitive data types: strings, numbers, Booleans, undefined, and null. Each of these data types serves a different purpose.

1. Strings are collections of alphanumeric characters and symbols used to store words and letters.
2. Numbers are simply numbers that are used to perform computer-based mathematical operations.
3. Booleans represent data that has two states such as true and false or on and off.
4. Undefined represents the creation of variables that haven’t been given a value.
5. Like undefined, null doesn’t have a value. However, the absence of a value in null is intentional.

Objects and arrays are also data types used in JavaScript. Both are collections and are used to compile multiple pieces of data into single bundles.”

Question #3: Describe What Hoisting Is in JavaScript.

Among other questions, you may be asked JavaScript interview questions about hoisting. This is a process commonly used in JavaScript coding. However, it’s best practice to only use it when necessary. Be sure to do the following when formulating your answer:

• Provide a brief definition of hoisting.
• Describe how it works in JavaScript.
• Explain the potential risks of hoisting and when to use it.

How You Could Answer

“Hoisting is an interpretation process in JavaScript coding. It occurs when declarations of variables, classes, and functions get moved to the top of the current scope or script just before the code gets executed. It allows programmers to use functions and variables before they’re declared.

Hoisting should be used with care. If a programmer doesn’t have a clear understanding of how hoisting works or when to use it, programs can end up with bugs and errors. You only want to use hoisting when you need to call functions or variables before they appear in code.”

Question #4: What Are the Main Types of Variables in JavaScript?

When a hiring manager asks any number of JavaScript interview questions about variables, provide a brief overview of what they are and how they work. Here’s the best way to answer:

• Describe what variables do in JavaScript.
• Identify the primary variable types.
• Explain the purpose of each variable type.

How You Could Answer

“Programmers use variables to store data values and contain data types such as Booleans, strings, objects, and numbers. JavaScript has three primary variable types: var, let, and const. Each of them serves different purposes in JavaScript programming.

The var keyword is often used to declare accessible and changeable variables. The let keyword allows programmers to declare variables that are restricted to the scope of black statements or expressions. The const keyword creates constants that are limited by block statements. Constants can’t be redeclared or changed through reassignment.”

Question #5: What Are Methods in JavaScript and How Do They Work?

Methods are an integral part of JavaScript programming. You can demonstrate your knowledge of methods in JavaScript by including the following details in your answer:

• A brief description of methods.
• Why methods are used.
• The convenience methods offer programmers.

How You Could Answer

“In JavaScript, methods are actions done on objects and properties that hold function definitions. Built-in methods are pieces of programming code that perform specific tasks. Programmers can easily call upon built-in methods to perform tasks without having to create new methods. Additionally, methods are convenient to use in JavaScript programming because they require less code, are reusable, and are easy to implement.”

Question #6: Tell Me About NaN in JavaScript.

Answering JavaScript interview questions regarding NaN is a great way to demonstrate your technical knowledge and competency in programming. When creating your answer, be sure to provide an overview of:

• What NaN is.
• What it represents.
• Why it’s used in JavaScript.

How You Could Answer

“NaN is an acronym for “Not-a-Number.” In JavaScript, NaN is a number that doesn’t represent a legal number and typically belongs to a global object. It’s a numeric data type that represents an undefined value. NaN is produced when a floating-point operation contains input parameters that create an undefined result such as 0 divided or multiplied by 0. It’s often used to check the validity of numbers in JavaScript programming.”

Question #7: What Is the Difference Between Passed By Value and Passed By Reference?

“Passed By” values and references are often used in JavaScript programming to make modifications. Both will be used often when coding in JavaScript. When an employer asks JavaScript interview questions about Passed By values and references, you can demonstrate your knowledge by:

• Defining both Passed By values and references.
• Describing their similarities.
• Differentiating between the two.

How You Could Answer

“Both Passed By values and Passed By references are used in JavaScript to call upon functions when making coding modifications. However, they have key differences.

For example, a Passed By value calls a function by directly passing the value of a variable as an argument. The original value remains unhindered by any changes made inside the function. A Passed By reference calls a function by passing the reference or address of a variable as an argument. Any changes made inside a function will impact outside variables.”

Question #8: Describe How to Change the Style and Class of an Element.

Styles and classes make up the layout and overall appearance of websites and apps. To be considered for a web or app development job, you’ll need to explain how you use JavaScript for front-end programming.

Create your answer by doing the following:

• Describe how JavaScript works on the front end.
• Discuss which methods programmers typically use.
• Provide a brief explanation of how to change elements.

How You Could Answer

“JavaScript is often used to change, add, or remove styles and classes from HTML elements. There are generally two ways to alter an element with JavaScript. The first is to set a CSS property directly on an element. This method involves accessing an element, finding a CSS property, and applying the appropriate text to give the CSS property a value.

The other way to alter an element with JavaScript is to add or remove class values. This changes which styles get applied. Like with the first method, you set a CSS property. But you also have to manage a list of class values that get returned as a string.

While the second method can be confusing and error-prone, JavaScript has new APIs that make it easier to create complex functionality.”

Question #9: What Is the “this” Keyword in JavaScript?

The “this” keyword is one of the most commonly used functions in JavaScript coding. Demonstrate your understanding of “this” by doing the following:

• Define “this” in JavaScript.
• Discuss why it’s used.
• Provide an example of what “this” refers to in JavaScript.

How You Could Answer

“The “this” keyword in JavaScript is a function that refers to a current object that is executing code. It’s often used to weed out confusion between class attributes and parameters that share the same name as a method or constructor. The “this” keyword also references a global object when a regular function is being executed. It references an object itself when the function involves a method within the object.”

Question #10: Which Types of Errors Might You Encounter in JavaScript and How Would You Address Them?

As a web or app developer, you’ve probably encountered JavaScript errors along the way. Employers will want to gauge your ability to identify errors and address them. JavaScript interview questions about errors also offer you a chance to demonstrate how to solve job-related problems.

Your answer should include the following details:

• The main error types.
• What causes these errors.
• How you fix these errors.

How You Could Answer

“Four primary types of errors exist in JavaScript: DOM-related errors, syntax errors, runtime errors, and logical errors. These errors are often caused by grammatical mistakes, unmatched brackets, and missing or incomprehensible text in the JavaScript code.

I typically use a JavaScript validator tool to identify errors. I’ll simply copy and paste the code or upload a JavaScript file and click on the “Validate JS” button. Many of these errors can simply be fixed by ensuring that the right code is in place.

For example, when a runtime error occurs, the syntax is trying to call a method that doesn’t exist. To fix this error, I would use the text code to create the method.”

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