25 Example JavaScript Interview Questions

JavaScript Interview Questions

JavaScript is among the most commonly used programming languages in the world today. In fact, according to Statista, 65% of software developers stated they use JavaScript while just over 56% stated they used HTML/CSS. Other common programming languages like Python and Node.js fell below the 50% mark. This is crucial knowledge for those who want to work in computer programming. In light of these figures, Java and JavaScript competency is crucial for employability.

How Common are JavaScript Interview Questions?

As one of the most commonly used coding languages with a global scope, JavaScript is a key technical skill for programmers. This means that JavaScript interview questions are likely to be far more common than questions about any other programming language in job interviews. Some of these questions may be asked and answered verbally, while others may come with a short practical task. Whichever question format you get, it is important that you take your time and give the best answer that you can.

25 Common JavaScript Interview Questions You Should Be Able to Answer

If you want to get a job interview for your dream job you will need a great resume, but once you are invited to an interview how you answer the recruiter’s questions is key. Consider these 25 sample questions and answers to help you to make a great first impression in your next JavaScript job interview.

1. What is hoisting?

   “Hoisting is a Java behavior that moves all of the function and variable declarations to the top.”

2. What is a type of operator?

   “A typeof operator is a function that returns a string description to the type of a specific local or global variable.”

3. Explain the meaning of NULL in JavaScript.

   “The presence or return of NULL in JavaScript indicates that there is a null string, no boolean value, and no array object.”

4. What are the five JavaScript data types?

   “The five JavaScript data types are number, string, boolean, null, and undefined.” Be prepared to provide a short overview of each data type as well as an indication of how they are used.

5. Explain the difference between a local variable and a global variable.

   “A global variable can be accessed globally, e.g. throughout the entire program. A local variable can only be accessed from within the function or block where it was defined.”

6. What is strict mode and how is it activated?

   “Strict mode'' is a helpful function that can be activated by adding the strong literal “use strict” above a file. When activated, strict mode highlights bad syntax by converting it into a real error in the system.” You will want to give an example, such as how a typo in a Java variable name can create a new global variable when strict mode is deactivated, whereas strict mode would highlight this typo as an error.

7. What is an anonymous function in JavaScript?

   “An anonymous function is a function which is not bound to an identifier. This makes it a type of nested function and therefore requires closures for implementation.”

8. Define syntax.

   “The syntax of JavaScript is a set of strict rules that define a properly structured program.” This may be followed by questions about syntax errors.

9. Define the meaning of const in JavaScript code.

   “In JavaScript code, the use of const indicates that an identifier cannot be reassigned. However, a const object can have its properties mutated which makes it distinct from immutable value objects.”

10. What is the console.log function in JavaScript?

    “This function displays log messages in the browser console.”

11. What is an undeclared variable?

    “An undeclared variable is a variable that doesn’t exist at all. This differs from an undefined variable that exists but is yet to be defined.”

12. What are JavaScript callbacks?

    “Callbacks are functions that are scheduled for execution after another function has finished execution. This prevents functions from running before necessary tasks have been completed.”

13. Define DOM.

    “The document object model, or DOM, is what defines the interface that allows programs access and lets them update the XML documents.” You can tie this into innerhtml for extra points. You might also be asked about inner function or how JavaScript can be used for HTML pages.

14. What are asynchronous codes?

    “Asynchronous codes are those that allow a program to be executed immediately, even if there is previous code being executed. Synchronous code requires that code being executed is completed before new code can be run.” This might lead to questions about the settimeout function, runtime, and runtime errors.

15. Explain the IsNaN function.

    “The IsNaN function registers a DataFrame object, true or false. It will return an object true for ‘not a number’ or NaN where all values are replaced with boolean values. Otherwise, it will return false.”

16. What does it mean to ‘debug’ in JavaScript?

    “The process of debugging involves locating errors and abnormalities in programs so that they can be repaired.”

17. What are JavaScript frameworks? How are they useful?

    “JavaScript frameworks are essentially libraries for code, often implemented using jQuery. They hold pre-written code for routine tasks. This can be really useful for new programmers or those looking to make routine changes to traditional programs.”

18. Explain the difference between function expressions and declarations.

    “The biggest difference between a function declaration and a function expression is the fact that a function name can be omitted in function expressions to create an anonymous function.”

19. Explain the function of the following code operator: ===

    “This is a strict code operator that will return true when two operands share a value without the need for conversion.”

20. Explain the function of VAR X.

    “VAR X is used to declare a variable, with X being the name of the variable.” You may be provided sample code that uses the VAR X function. Explain the role of VAR X and explain the VAR keyword.

21. What is a constructor?

    “A constructor creates a new object or sets values for existing object properties.” You can also discuss JavaScript objects as a whole.

22. How would you merge strings?

    “I would use the concatenation or concat() method to merge strings and create a single, larger string.”

23. What are function parameters and arguments?

    “In the JavaScript scripting language parameters are a part of the function definition while arguments are values that are passed to the function once it is invoked.”

24. When should arrow functions not be used?

    “Arrow functions are not regular functions and they shouldn’t be used for object methods or callback functions. They can reduce the readability of code, but provide a shorter syntax for function declarations.” This question may be followed by follow-up questions about parameters, or other advanced topics.

25. What is the difference between foreach loop and for loop?

    “A foreach loop processes each element in a wider collection in turn while a for loop can work outside of collection elements.”

If you want to be a JavaScript developer, these are the kinds of questions you should expect in a job interview. Questions like this are intended to assess your JavaScript programming knowledge and skills, and potentially your knowledge of programs that JavaScript supports. Your knowledge will be the center of a programming job interview, but you should still consider these general interview tips to ensure you make a great impression.