Top JavaScript Interview Questions and Answers for 2021

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The interview is one of your job’s central parts, directly or indirectly affecting your successful career.

As a programmer, web developer, or engineer who intends to be a part of a software house or wants globally connected to the web developers, you must know all the JavaScript interview questions I defined in detail. I tried my best to explain and clear all your concepts related to JavaScript and discuss them in detail. So you can easily face the top JavaScript interview questions.

Most of the students want to be freelancers, and many of them are related to programming and computer-related fields. For those students, advanced JavaScript interview questions are the most important questions because they are clear with their basic concepts. They must have to get advanced concepts to become professional.

Therefore, the javascript interview questions to top javascript interview questions and javascript interview questions to advance javascript interview questions are for beginner to intermediate level and then to be a professional.

Javascript Interview Questions
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Some basic JavaScript interview questions:

Q1:  What is Javascript?

Ans: Javascript is one of the scripting language widely used in client-side validations. It is a different language from java and also objects based lightweight and cross-platform translated language.

Q2: What is mean by scripting language?

Ans: Scripting language means interpreted. It is a programming language that translates into machine code after compilation(code is run).

Q3: Can you tell me some features of JavaScript?

Ans: Features of JavaScript are:

  • Interpreted programming language
  • Compatible with java
  • Compatible with Html
  • Lightweight
  • Open-source cross-platform

Q4: What are the advantages and dis-advantages of JavaScript?

Ans: There are many advantages and disadvantages, but some of them are:


1: Interactivity is high

2: Interfaces are richer

3: Feedback to the visitor is immediate

4: Server interaction is less


1: There is no support for multithreading

2: There is no support for multiprocessing

3: There is no reading and writing of files.

4: Restriction for networking application support.

Q5: What are the names of types of functions?

Ans: The types of functions are:

1) Named: Named types of functions contain names at the time of definition.

2) Anonymous: These functions do not include any name because they declare dynamically at runtime.

Q6: Tell us, Can we assign an anonymous function to a variable?

Ans: Yes, we can assign

Q7: What is an argument object in JavaScript?

Ans: JavaScript variables represent the arguments that are passed to a function.

Q8: Define closure?

Ans: In JavaScript, we need closures when a variable defined out of scope in reference is accessed from some internal scope.

Q9: Can you tell me the difference between JavaScript and Jscript?

Ans: Netscape provided the JavaScript. But, Microsoft changed the name from JavaScript to Jscript to avoid trademark issues. We can say that JavaScript is the same as Jscript but provided by Microsoft.

Q10: Is JavaScript case sensitive?

Ans: Yes, JavaScript is case sensitive language.


Q11: How to create an Array in JavaScript?

Ans: By these three ways

1) By Array literal

2) By creating an instance of an Array

3) By using an Array constructor.

Q12 : Tell me what is isNaN() function ?

Ans: isNaN() function return true if the variable value is not a number

Q13: What is the output of 10+10+”21″ in JavaScript?

Ans: The output will be 2021 because if there is a numeric value before and after +, it treats as binary so, 10+10=20 and the “21”.

Q14: What is the output of “10” +10+21 in JavaScript?

Ans: The output will be 101021 because, after the string, all + will be considered as string concatenation operator, not a binary.


Q15: Do you know the difference between client-side JavaScript and server-side JavaScript?

Ans: Client-side: It comprises the primary language and predefined objects relevant to running JavaScript during a browser. It is embedded directly in the HTML pages and the browser interprets this script at runtime.

Server-side: Server-side JavaScript also resembles client-side JavaScript. It has relevant JavaScript, which is to run during a server. The server-side JavaScript are implemented only after compilation.

Q16 : How can you define the difference between == and === operators ?

Ans: The ” == ” operator only checks the equality, and on the other hand, the ” === ” operator checks the equality and data type also.

Q17: What are the pop-up boxes available in JavaScript?

Ans: Alert box, Confirm box, Prompt box.

Q18: Which string we use to detect the OS of the client machine using JavaScript?

Ans: We can use navigator.appVersion string to detect the OS on the client machine.

Q19: JavaScript faster or ASP script? If JavaScript, then why?

Ans: JavaScript is faster because JavaScript does not require the web server’s support for execution.

Q20: What is the requirement of debugging in JavaScript?

Ans: JavaScript did not show any error messages in a browser. However, these errors can affect the output. The best practice to discover the error is to debug your code. The code can be easily debugged using web browsers like Google Chrome, Mozilla Firefox.

For debugging, we can use any of the following approaches:

1: Using console.log() method

2: Using debugger keyword

Q21: What’s the role of strict mode in JavaScript?

Ans: JavaScript strict mode is used to generate silent errors. Provides “strict use”; expression to enable strict mode. This expression can only be placed because of the first statement during a script or a function.

Q22: What is the use of Map object in JavaScript?

Ans: The JavaScript Map object is used to map keys to values. It stores each element as a key-value pair and operates the elements such as update, searches, and delete based on specified key.

 Q23: What is the use of the WeakMap object in JavaScript?

Ans: The JavaScript WeakMap object is a type of collection that is almost similar to Map. It stores each element as a key-value pair where keys are weakly referenced. Here, the keys are objects, and therefore the values are arbitrary.

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