JavaScript Interview Questions

Because JavaScript is one of the most popular programming languages in use today alongside HTML and Python, JavaScript interview questions are fairly common when you’re applying for a programming or development job. This means that preparing to answer them is a must for those who want to excel and stand out from the crowd.

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There are some general, non-coding, JavaScript interview questions you might get asked in a job interview, both about Java and about the internet as a whole. For example, you might get asked questions about JavaScript code as a programming language, about the differences between scripting languages and programming languages, or about how the back end and front end of a web page interact. You could even be asked about operating systems like Microsoft. You are also likely to be asked about how you’ve used your JavaScript knowledge and skills in the past.

There are some specific JavaScript interview questions that you might be faced with if you apply to be a JavaScript code developer or a programmer. Here are some examples of the common interview questions that could come up in an interview: 

1.  What are the JavaScript data types?
“The JavaScript data types are number, string, boolean, object and undefined.” (You will probably be expected to give a brief overview of each kind).

2. Explain hoisting within JavaScript.
“Hoisting is a behavior of Java whereby all the function and variable declarations move to the top.” (You may also be expected to explain local variables and global variables)

3.What is Node.Js?
“Node.js is a JavaScript runtime that was built on Google Chrome’s V8 engine.” (You may also be asked to give a brief overview of its features). 

4. What is the IsNaN function?
“The IsNaN method or function returns a DataFrame object true for ‘nan’ or not a number value where all values are replaced with a boolean value. Otherwise, it returns false.”

5.  What are undefined and undeclared variables?
“Undeclared variables are those that do not exist in a program and are not declared. Undefined variables are those that are declared but which have not been given a value.” (This question may tie into global variables and the details of function declaration as well as function names or function scope).

6. What does NULL mean in Java?
“NULL represents no value or no object. It implies that there is a null string or no object, no boolean value, or no valid one at least, and no array object.”

7. What is Strict Mode and how can it be enabled?
“Strict Mode changes accept bad syntax into real errors, thereby highlighting problems. For example, in normal Java, mistyping a variable name would create a new global variable. In Strict Mode, this is not possible because it will throw a reference error. Strict Mode is enabled by adding the strong literal ‘use strict’ above the file in question.”

8.What is a type of operator?
“The type of operator is used to return a string description to the type of a global or local variable.”

9. When should you not use arrow functions in JavaScript?
“Arrow functions provide us with a new, short syntax for declaring functions. They do not replace regular functions and should not be used for object methods, callback functions, or when they generally reduce the readability of the code.” (You may also be asked about parameters, object literals, or other features that tie into arrow functions. This would be a fairly advanced JavaScript interview question.)

10.  What is an anonymous function in JavaScript? 
“An anonymous function is one that is declared without any named identifier. These are generally inaccessible after their declaration.” (You may also be expected to discuss the importance of variable name or function expression in your answer.)

11. Explain negative infinity.
“Negative infinity is a number that, in JavaScript, can be found by dividing a negative number by zero.”

12.What is debugging in JavaScript?
“Debugging is the routine process of locating and removing errors or abnormalities from programs”

13. Explain operator.
“=== is a strict equality operator. This means it returns true when the two operands have the same value without the need for conversion.” 

14.What is the difference between local scope and global scope?
“Global variables are useful for relatively constant values, while local variables are declared within a function block. Global scope encompasses the whole program. Thus, any change in global variables impacts the whole program. Local scope, by contrast, is limited.”

15.What is a constructor in JavaScript?
“A constructor is a feature that creates a new object and sets a value for any existing object properties. (You may also be asked more about JavaScript objects or object-oriented programming.)

16.What does const mean when placed in code? 
“In JavaScript, ‘const’ means that the identifier cannot be reassigned. This is not the same as immutable values, because, unlike immutable data types, a ‘const’ object can have its properties mutated.”

17. Explain VAR X here. (can be asked about any segment of code)
(Explain the function of the highlighted feature or code, you may also be asked more about the VAR keyword).

18.  How do you merge strings in JavaScript?
“You can use the concatenation or concat() method to merge strings. The concatenation operator allows for the merging of the contents of two strings to create one larger string.”

19.  What is the console.log function?
“The console.log function is used to display log messages to the browser console.”

20.What is DOM in Java?
“DOM or the document object model is an official recommendation of W3C. It defines the interface that lets programs access and update XML documents.” (This may tie into things like inner html).

21.What is asynchronous JavaScript?
“Asynchronous codes allow a program to be executed straight away whereas synchronous codes would block the execution of the code until the completion of the code currently being executed.”

22.Explain SAX. 
“SAX or simple API for XML is an event-based XML parser. This doesn’t load a whole XML document.”

In addition to these specific JavaScript interview questions. you may also be asked things like “explain the following code” or be asked to demonstrate or explain concepts as simple as initialisation or iteration to show proficiency with a supplementary tool like JQuery. You may also be asked to complete short, timed tasks using the scripting language. If you know that there are areas of weakness in your Java knowledge, watch some tutorials in those areas before your interview to refresh your knowledge. 

Depending on what kind of experience you have and the positions you have previously held (as well as whether or not you know people in the company you are applying to), you may be asked to prove that you are Java proficient. If this happens there are a number of possibilities. First and foremost, you may be asked specific questions about how JavaScript works and how you have used it in the past. Secondly, you may be presented with a piece of code and asked what the output will be when it runs. You could also be asked questions about a web application, or coding as a whole. Finally, there is a chance that you could be given a small coding assignment to test your proficiency. While this may seem daunting, especially if you are applying for your first job in the field, you can strategise for these kinds of questions in the same way as you would for other interview questions. CVHelp has a wealth of interview preparation tips that could be helpful to you!

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