Top 10 JavaScript Interview Questions and How To Answer Them

Developed in 1995, JavaScript is now the universal language of web developers. This programming language is supported by all the most popular web browsers. In addition to Internet Explorer, Chrome, Safari and Firefox. Furthermore, some of the less-popular browsers, such as Opera and Edge. JavaScript is even supported by smartphones, tablets and other mobile devices.

JavaScript makes everything easier for internet users. Thanks to this language, web developers are able to design sites that are user-friendly, dynamic. Specifically, include key features, such as dialog boxes, menus, animations, video players, interactive maps and much more.

In short, JavaScript is essential to internet use as the world knows it. For this reason, it is imperative that developers not only understand JavaScript basics. Particularly, that they also master the most complex JavaScript functions. Moreover, it is important that aspiring web solutions programmers can answer the top JavaScript interview questions in a fluent and in-depth manner. If you hope to land a job with a reputable company as its developer. Considering, to prepare for the top 10 interview questions that you will find in this article.

What is the reason for wrapping the entire content of a JavaScript source file in a function block, and why is it important?

Popular JavaScript libraries use this practice to create closure around a file’s entire contents. The purpose of doing this is to create a private namespace. Especially, can help avoid name clashes between several different JavaScript libraries and modules. This technique also allows you to create an easily referenceable alias for a global variable. Thus, you can perform different functions by using this short piece of code.

Why would you use a ‘use strict’ at the beginning of a JavaScript source file?

There are several different benefits to using a ‘use strict’ code at the beginning of a source file. However, the greatest advantage of doing so is that it provides a way to voluntarily enforce stricter error handling and parsing on the code at the time of implementation.

Code errors that would have otherwise gone unnoticed will now produce errors or generate exceptions. Moreover, in general, a good practice. This is the main reason that website owners appreciate developers who use this function.

There are other key benefits of using a code. Those include the following:

  • It prevents accidental globals. If you don’t write in ‘use strict’ code, you may accidentally assign a value to an undeclared variable, which will automatically create a variable in that name. In a strict context, attempting to do this will produce an error.
  • That simplifies the debugging process. If your code has an error, the strict operating context will make finding it much faster and simpler. That will also direct you to the source of the problem, rather than making you guess where it’s at.
  • Design that made impossible to create a duplicate parameter value. If you create a duplicate named argument for a function, strict mode will produce an error alert.
  • Creates eval() less risky. Eval() behaves differently in strict mode and non-strict mode. One of the biggest risks you take when using eval() in non-strict mode is that variables and functions created within it are created in the containing scope. This often becomes a source of headaches.
  • It brings impossible to use an invalid delete. Delete is used to remove properties from objects, but if you use it incorrectly. Otherwise, you may delete a non-configurable property. If you attempt to use delete on the wrong property in strict mode, an error report will be generated.

What is the difference between undefined and non-defined in JavaScript?

“Non-defined” refers to a variable that does not exist, whereas “undefined” refers to a type of undeclared value. If you try to use a non-defined variable, the system will throw an error.

What is the main drawback of using true private methods in JavaScript?

The main drawback of using these types of methods is that they are memory-inefficient. If you do use them, your program would create a copy of the method for each instance.

What are two programming paradigms that all JavaScript app developers should know?

Prototypal inheritance and functional programming are basic knowledge. Though JavaScript is a multi-paradigm language, employers like to hear that potential developers value these two essential functions.

What is functional programming?

Functional programming refers to the process of composing mathematical functions and avoiding shared or mutable data. This is one of the most essential features in JavaScript and helps to effectively do the following:

Compose simple functions

  • Avoid negative side effects or unintended consequences
  • Ensure function purity
  • Support other essential JavaScript features

What is the difference between classical prototypal inheritance and classical inheritance?

In instances of prototypal inheritance, instances inherit directly from other objects and may be composed of many different objects. Prototypal inheritance allows for simplified selective inheritance.

Classical inheritance refers to instances that inherit directly from classes, such as blueprint, and is essential for creating sub-class relationships. Instances operating with classical inheritances utilize the “new” keyword and may or may not require the “class” keyword.

When might you use classical inheritance?

You almost never want to use classical inheritance, especially if you have more than one level. Classical inheritance comes with many risks. Thus, most of which developers and website owners alike are unwilling to take. In short, the best answer is simple: if there is a better way, use it.

When is it appropriate to use prototypal inheritance?

It depends on the type of prototypal inheritance to which you are referring. There are three types of prototypal inheritance you can use:

  • Functional: Developers use this function to generate a closure for private state/encapsulation. It is not to be confused with functional programming.
  • Delegation: Delegation refers to the prototype chain.
  • Concatenative: Examples of concatenative would be mixins and object.assign().

 Each of these types of inheritance is equally useful when implemented correctly. Depending on what it is used for, it can create a has-a, can-do or uses-a relationship.

Describe one-way data flow and two-way data binding and what distinguishes the two from one another.

One-way data flow refers to a model utilizing a sole foundation of truth. If you try to make changes to UI fields, the UI generates messages that indicate user intent to the model.

In one-way data flow, only the model has access to the fields and the ability to make changes. The result is that the data only flows in one direction. These types of setups are easier to comprehend and therefore easier for newbies in the development world.

Two-way binding means that the UI fields will be more dynamic and that users can make changes to the model data. Additionally, the model can make changes. However, two-way binding can result in side effects that are difficult to pinpoint and even harder to understand.

You Will Need To Prove Your Knowledge of Formulas

In addition to needing to know the direct answers to the aforementioned 10 javascript interview questions, you will also need to demonstrate your knowledge of certain formulas. Depending on what position you are applying for and how much experience you are expected to have. Due to the fact that these formulas can range in complexity from basic to comprehensive. You will be expected to know what the output of a certain code will be and demonstrate how to create lines of dialogue intended to achieve a certain output.

Javascript Interview Questions

Additional Questions You May Be Asked

Once you prove your knowledge, the interviewer may want to get a little more personal with his or her questions. For instance, he or she may ask why you favor JavaScript. If you don’t favor JavaScript but simply want a job, the interview is not the time to make your feelings known. Some smart answers would be along the lines of the following:

  • You can write directly in the browser and run it everywhere.
  • You can create closures with ease.
  • It’s easy to get started and, once in, you can write in various other languages.
  • Nearly everything is a (mostly) mutable object.

The thing that most people enjoy about JavaScript, however, is the ability to do just about anything with the language. It is taking over the world, a fact that most developers thoroughly enjoy. Some other cool things you can do with JavaScript include the following:

  • Store and retrieve values
  • Write event handlers to respond to other events and users
  • Define your own classes
  • Define and invoke functions
  • Declare variables
  • Load and use external modules
  • Much, much more

JavaScript Is the Programming Language of the Future

JavaScript is by far the easiest and most universal of all the programming languages. The simplicity and relevance are the top two reasons that developers use it.

More importantly, why employers prefer developers who use it. If you hope to land a good developing job, you need to be fluent in JavaScript and able to convey that fluency to someone who is likely not familiar with the language. This can be tough to do, but, with the right amount of practice, you can answer your interview questions with knowledge and confidence.

Featured Image: CCO Public Domain by Bo-Yi Wu  via Pixabay


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