JavaScript interview questions and answers for 2023

Are you a JavaScript developer seeking a job in one of the top US MNCs? Or, are you a recruiter from a top US MNC looking for an excellent JavaScript developer? In either case, you have landed on the right page. This Turing collection of JavaScript interview questions and answers will help you to net the best job or the best candidate.

Last updated on Mar 11, 2022

JavaScript is a popular programming language that is frequently used by the likes of Google and Facebook for their applications. So, if you are a senior JavaScript developer, you know that you have opportunities galore with this skill. However, to crack a JavaScript interview as a developer at a top US MNC, you need to brush up on JavaScript interview questions. We have collated advanced JavaScript interview questions to help you get on track.

It doesn’t matter if you are a candidate looking for a JavaScript job or a recruiter looking for a JavaScript developer, these JavaScript interview questions can help you.

Advanced JavaScript interview questions and answers


Describe Arrow functions.

The ES6 Javascript version introduced Arrow functions. With the Arrow functions, we can declare functions using new and shorter syntax. These functions can only be used as function expressions. The declaration of these functions is done without using the function keyword. Moreover, if there is a single returning expression, then even the return keyword is not needed. Additionally, wherever the code occurs in a single line only, we can omit the curly {} braces. If there is only one argument in a function, then we can omit even the () parenthesis.


Why are Arrow functions used almost everywhere?

Arrow functions are used everywhere because:

  • Safety of scope: When the arrow function is used everywhere, it brings consistency of scope because the same thisObject is used everywhere. If by any chance, a standard function is used alongside the arrow function, there are chances of the scope getting mixed up.

  • Compactness: As compared to the standard function, the arrow function is compact as it does away with the need for keywords, curly braces, parenthesis, etc. in certain cases. It is, therefore, easier to read and write.

  • Clarity: When the arrow function is used everywhere, there is a certain consistency of scope. Thus, whenever a standard function is mixed with it, it stands out distinctly. The developer can therefore look for the next higher function to locate the thisObject.


When should generators be used in ES6?

Generators in ES6 can be used in two main scenarios:

  • When one wants to move out of a function, one can do so using generators and the outer code determines when to move back into the function
  • With the help of generators, one can control an asynchronous call outside the code.

Most importantly, though, the next value can come in only when it is needed; all values do not need to come back in at once


Explain rest parameter and spread operator.

The ES6 version of Javascript introduced the rest parameter and spread operator.

Rest Parameter

The use of three dots (...) before any parameter shows a rest parameter. This improves the handling of function parameters. With the help of the rest parameter, we can create such functions that can take a different number of arguments. By using the rest parameter, all these arguments will be converted into arrays. The rest parameter can also be used to extract any or all parts of an argument.

Spread Operator

Though the spread operator syntax is the same as that of the rest parameter, the spread operators help to spread arrays and object literals. Another use of spread operators is when one or more arguments are expected to be in a given function call. So, while the rest parameter creates an array, a spread operator spreads an array.


Why are promises used in JavaScript?

Promises help in managing asynchronous operations such as server requests in JavaScript. Earlier, callbacks were used for the same purpose. However, callbacks have limited functionality and thus, can make the code unmanageable. There are four states of the promise object:

  • Pending: This is the first state of the promise and shows that the promise has been neither fulfilled nor rejected.
  • Fulfilled: This state represents the completion of the asynchronous operation. In other words, the promise has been fulfilled.
  • Rejected: This state represents the failure of the asynchronous operation due to some reason. In other words, the promise has been rejected.
  • Settled: This is the last state of the promise, showing that the promise has been either fulfilled or rejected.

A promise constructor uses a callback with two parameters - resolve and reject - to create a promise. The resolve function is called when the asynchronous operation has succeeded. The reject function is called when the asynchronous operation has failed.


What are classes in Javascript?

Classes in Javascript are templates for building objects. Classes bind the data with code so that the data works as per the code. They were introduced in the ES6 version of Javascript and while they were created on prototypes, they also have syntax and semantics that are not common with ES5. Classes can be seen as special functions. There are two components of class syntax: class expressions and class declarations.

Class expressions are one of the ways to define classes. They may or may not have names. If a class expression has a name, it is held locally in the class body but can be accessed through the name property. Before using class expressions, one must declare them.

Another way to define classes is class declaration. For declaring a class, the class keyword must be followed by the class name.

One class may use the properties of methods of another class by using the extend keyword. Classes in Javascript must follow the strict mode. If the strict mode is not followed, errors will appear.


What is WeakSet in Javascript?

WeakSet in Javascript is a collection of unique and orderly objects. Unlike Set, a WeakSet does not contain any other elements. Those objects of a collection that are weakly held, appear in WeakSet. This also means that if there is no reference for a weakly held object, it will be collected as garbage. There are three methods of WeakSet: add(), delete(), and has().


Explain temporal dead zone.

Temporal dead zone (TDZ) was introduced for the first time in the ES6 version of JavaScript. TDZ is a behavior that occurs when a variable is accessed before it is initialized. It is a language construct that helps to catch errors as accessing data before initializing it is incorrect. When let and const keywords are used to declare variables a TDZ may occur.


What is object destructuring?

Object destructuring is a JavaScript feature to extract object properties and bind these properties to variables. It can be used to extract several properties in a single statement and can reach properties from nested objects. When no property exists, object destructuring can set default values.


What is WeakMap in JavaScript?

WeakMap object stores key-value pairs with weakly referenced keys. Keys in WeakMap must only be objects, while values can be arbitrary. Primitive data types cannot be keys in WeakMap. Since native WeakMap contains weakly referenced keys, these keys can be garbage collected and therefore references get removed. Also, because of the weak referencing, garbage collection of values in WeakMap is not prevented. When information about a key is valuable ‘only if’ the key is not garbage collected, WeakMap is useful in mapping keys to the information about them.

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Wrapping up

Now that you have gone through some important JavaScript interview questions and answers, you have some idea of what to expect in the interview. However, your JavaScript interview is not made up of technical JavaScript interview questions. You will most likely be quizzed on your life and social skills. The recruiter will try to ensure that you are not just a brilliant JavaScript developer, who can ace JavaScript interview questions, but a compassionate team player who understands the pace and difficulties of the challenging work environment. You must ensure that you prepare for such questions in addition to the JavaScript interview questions.

If you think you have it in you to make it to top US MNCs, head to Turing’s website and apply for Jobs. If you are a recruiter at a top US MNC looking for the best JavaScript developers, contact Turing today.

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