Junior FrontEnd JavaScript Developer Interview Questions

Since 2014, JavaScript’s popularity has been on the rise. And so has the number of people learning it which means more and more juniors apply for JavaScript Developer positions. But don’t worry, the job openings are also on the rise.

Applying for junior developer positions can be scary and intimidating. You don’t really what to expect of it, what knowledge you’ll be supposed to have, how helpful your team will be and if you’ll fit it and be able to learn more as time goes by.

Generally, an interviewer looks for a candidate that knows the basics and has already practiced a bit. Personal projects are always great for this. Understand very well what this project of yours is about, what you did for it and why you did things the way you did. When talking about it, the interviewer will have the opportunity to test your thought process as well as your actual technical knowledge.

I’ve noticed that some questions come up regularly, so here’s a list of them I made.


Questions and answers

What is the difference between == and ===?

The == operator compares two values, without taking into account their types. === compares the values, but also makes their types into account. So 3==’3’ evaluates to True, but 3===’3’ evaluates to False.


What is the difference between textContent and innerHTML?

textContent is a concatenation of the text content of a node and its descendant. It is the fastest of the two.

innerHTML returns the HTML. It parses the text into HTML, which makes it slower.


What is the DOM?

DOM stands for the Documentation Object Models, and it is a World Wide Web Consortium standard.It defines a standard for accessing documents, and it can be used to access and change the content of the HTML.


What are the JavaScipt data types?

There are seven data types: Boolean, Null, Undefined, Number, String, Symbol, Object. The first six are primitives, meaning data types that aren’t objects and do not have any methods.


What are the pop-up boxes types available in JavaScript?

There are 3 pop up boxes: Alert, Confirm and Prompt.


What do the break and the continue statements do?

The break statement is used to exit a current loop. On the other hand, the continue statement continues with the next announcement in the loop.


What is negative infinity in JavaScript?

The result of a negative number divided by 0.


Explain the differences between var, const and let.

The weakest signal available in ES6 is var.

‘Let’ signals that a variable might be reassigned, and can be used for example in loops, for counting.

‘Const’ signals that that identifier will not be reassigned.


Give us an example of an object declaration.

Var person = {

fullName: ‘Andrew Smith’,

Age: 34,

Email: ‘[email protected]



Explain the difference between null, undefined or undeclared variables.

Undefined variables have been declared, but no value exists for them. Null is a value of variables, as well as a type of object. Undeclared variables are those declared without the var, const or let keyword.


General advice and conclusions

Depending on the framework the company uses, you’ll most probably have lots of questions about it. It is recommended to have good knowledge of one significant framework such as React, Angular or Ember.

Know how objects, classes, inheritance work in JavaScript and be able to give a reasonable explanation. Also, understand how functions and objects relate in JavaScript.

Be up to date with ES6 concepts. Know what for…of and for…in loops do, what are the newest Array methods such as map and filter. And more importantly, explain how they work. The setting up is not as important as understanding it, knowing how it works and what is dows.

You should be able to manipulate the DOM and change how it looks based on events, be at least a bit familiar with JQuery and Bootstrap, have plenty of knowledge about HTML and CSS and know how to debug JavaScript with the developer tools provided by browsers.

All in all, during interviews, the employer will probably be more interested in your problem-solving skill, in the way you can explain how JavaScript works, not just how awesome you’re at remembering the exact syntax or mdn’s definitions.

Always be learning new things. This way, you’ll not only get your job as a junior, but you’ll move to a mid position in no time.

Diana Caliman

Diana Caliman