Here are the 20 JavaScript questions you’ll be asked in your next interview

An interview is an important part of the hiring process. It gives insights about the skillset, knowledge, and the ability of an individual to take on challenging tasks. It’s also one of the most reliable ways for an employer to filter out unsuitable candidates for a job posting.

As a JavaScript developer, you must prepare yourself before appearing in an exam or an interview. It’ll increase your chances of getting hired by a reputed company.

Now, you might be wondering where to start when it comes to preparation and getting familiar with the kind of questions that’ll be thrown your way. So, to help you out, I’ve compiled a list of commonly asked JavaScript interview questions. Each of these questions has a brief answer which you can review below.

In the end, you’ll be a bit more prepared and confident to answer any question your interviewer may ask. So, let’s get started.

First of all, what is JavaScript?

JavaScript is a scripting language created by Netscape in 1995. It was initially used on a popular web browser called ‘Netscape Navigator. ‘But, these days, we can use it for client-side as well as server-side application development.

Well… I don’t think someone will ask this, but we shouldn’t forget where JavaScript comes from.

Can you list some advantages of JavaScript?

JavaScript can work offline inside the web browser.

It supports multiple programming paradigms. For example, we can make use of Object-Oriented, Functional, and Imperative programming concepts.

It has the biggest collection of open source libraries and frameworks.

JavaScript is capable of creating online and offline games, desktop software, websites, and mobile apps.

No need to learn separate programming languages to create frontend and backend of a website. JavaScript is supported on all major web browsers and it can run on the server using Node.js.

It is an interpreted language. Meaning we don’t have to build or compile its code before use. JavaScript instructions execute directly.

Should we use internal or external JavaScript?

Internal JavaScript is more suitable when we just need to use it on a single web page. Whereas, always use an external JavaScript file for websites that have multiple web pages.

Why do WordPress plugins like Autoptimize aggregate JavaScript Code in one file?

Aggregation of JavaScript source code in a single file reduces the number of requests made to the server while generating a web page. In turn, it makes a website load faster.

For example, let’s say we’ve included ten JavaScript files on a web page. Now, when we open this web page our web browser sends ten HTTP requests to the server in order to retrieve these files. On the other hand, if we aggregate the code of all these files into one then we just need to make one request to the server.

What is JavaScript ‘hoisting?

Hoisting is a concept in JavaScript which allows us to use variables and functions even before they are declared.

Basically, when we execute a JavaScript code then at first it automatically extracts all variable and function declarations from the code and moves them to the top of their scope. After that, it starts executing the code.

The major benefit of hoisting is our code works correctly and doesn’t display any errors like “undefined variable” or “undefined function.”

Learn more about hoisting with my article ‘ Understanding Variables, Scope, and Hoisting in JavaScript .’

Can you predict the output of the following code?

Code:

Output:

Explanation:

First of all, the variable declaration at line # 2 and line # 3 will be considered as one statement.

Now, the concept of hoisting will be applied. Meaning JavaScript will move the variable declaration to the top. After that, the code will be executed.

Also, remember that the value stored in a variable will not be lost even if we redeclare the variable.

What is JavaScript ‘Strict Mode’?

The default behavior of JavaScript is very forgiving in case we make a minor mistake. It means it will not display any error messages. But, sometimes in development, we need to see all kinds of errors and warnings to debug the code.

Here comes the use of “Strict Mode” in JavaScript. Basically, it’s a restricted variant where JavaScript displays all errors and warnings, even if they are silent ones.

We can enable “Strict Mode” by using the "use strict"; directive at the beginning of our script.

What are some alternatives to Svelte?

Svelte is a front-end development framework for the JavaScript programming language. Some of its popular alternatives include:

React

Vue.js

Angular

What are ‘Self Invoking Functions’?

Self Invoking Functions are a little different from normal functions because they are executed immediately where they were declared.

Normally, we first declare a function and later just call it. But, JavaScript automatically executes the code of Self Invoking Functions at run-time.

A point to be noted is that these functions do not have any name. In turn, we are unable to recall these types of functions. They are also known as “Anonymous Functions”.

Here’s an example of Self Invoking Functions:

What is the difference between ‘var,’ ‘let,’ and ‘const’?

Learn more about them in my article ‘ Understanding Variables, Scope, and Hoisting in JavaScript .’

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

Both of them are used in JavaScript to perform a comparison between two values.

Is there any difference between ‘null’ and ‘undefined’ keywords?

Both of these keywords represent an empty value . But, there are two basic differences between null and undefined .

Can you tell the difference between ‘function declaration’ and ‘function expression’?

Basically, ‘Function Declaration ‘ is nothing but the normal process of defining a function using the keyword function , its unique name , parameters, and the function body. Whereas, when we assign a function declaration to a variable then it becomes ‘Function Expression.’ It’s interesting to note that function expressions are not hoisted, meaning, they’ll display an error if you try to call them before defining.

Function Declaration:

Function Expression:

What is a ‘Closure’?

A ‘closure’ in JavaScript is a function inside another function. The inner function has access to its own variables, the variables defined in the outer function as well as the global variables.

Closure Example:

Can you predict the output of these two functions? And will they return same the output or not?

Output of Function user1()

Output of Function user2()

What is ‘NaN’?

In JavaScript, NaN stands for “Not a Number.” It’s a special value that occurs when we’re unable to perform an operation.

For example, what if we try to divide a string using a number (e.g. “Hello World” / 5).

Can you explain the for-in loop?

The for-in loop is specifically designed to loop through all the object’s properties in a step by step manner. It selects one property from the object in each iteration and performs the required operations on it.

Let’s try to understand it with the help of an example:

Output:-

What’s ‘Event Bubbling’ and ‘Capturing’?

In JavaScript DOM, HTML elements are nested inside one another to form a hierarchy.

Now, if both parent and child elements have registered a handle for a specific event then what will be the order of event propagation?

It can be determined in two ways which are known as event bubbling and capturing.

In event bubbling, the child element will capture the event first and then propagate it to parent elements. Whereas, in event capturing, the parent element will capture the event first and then propagate it to the child elements.

What’s the difference between JavaScript and ECMA Script?

JavaScript is a scripting language whereas ECMA Script is a collection of guidelines and rules to standardize JavaScript across different web browsers.

How do you create a cookie using JavaScript?

In JavaScript, a cookie can be created using a documenookie object. Simply assign it a string value that is just a series of key-value pairs separated with semi-colons.

JavaScript Create Cookie Example:

Conclusion

Employers usually try to confuse the applicants by asking tricky questions. So, if you’re not well-prepared then chances are you’ll end up losing the opportunity.

So, today, I tried to answer some commonly asked JavaScript interview questions. You may even use it as a reference just before going to an interview.

This article was originally published on Live Code Stream by Juan Cruz Martinez (twitter: @bajcmartinez ), founder and publisher of Live Code Stream, entrepreneur, developer, author, speaker, and doer of things.

Live Code Stream is also available as a free weekly newsletter. Sign up for updates on everything related to programming, AI, and computer science in general.

How I grew my third startup faster than ever before

It was six times quicker to launch Rnwl , my current startup, than my previous one. The level of complexity, product novelty, and financial requirements are particularly similar between the two. Yet this time it took me just four months to build the product, the team and the funding to a satisfactory level whereas it took nearly 24 months to get to a similar position with my previous startup, SyndicateRoom (SR) .

What was it that allowed me to deliver so much faster? And why is it that serial entrepreneurs seem to execute far faster than they did their first-time around? Sure, experience is key. But what is it that serial entrepreneurs do differently when building a new company? More importantly, how can it be replicated by first time entrepreneurs?

Below are the top five things I am doing differently this time around as I build my latest InsurTech startup Rnwl, which focus on insurance renewals . The good news is that they can mostly be replicated by first-time entrepreneurs. The bad news is that I have no idea whether it increases the chance of success — but I think it’s worth a shot.

1) Read. A lot.

I stepped down from the role of CEO of SR last Summer. As it happens, it was a great summer weather wise and I made the best out of it by reading out in the garden. And I read a lot. And I mean a lot. I read on the matters I felt I needed the most. I now find myself using the knowledge gained on a daily basis and revisiting some of the books every now and then, particularly the marketing ones.

Spending time reading may sound like a frivolous waste of time when you are itching to build a company, but nothing could be further from the truth. Most of the important details behind Rnwl came to my mind as I was reading. The time spend reading before starting to work on building Rnwl gave me time to learn and think. To plan. To prepare to execute quickly. These are trying times , but try to make the most of it and lay to foundation to realizing your vision.

2) Hire an experienced founding team

Never underestimate the difference it makes to have an experienced founding team. You should hire the very best founding team you can afford. Attitude and personality will only take you so far. Having an experienced team with the right attitude and personality will help delivering better quality results much quicker and both are crucial if you want to survive the early stages of a company.

Unfortunately, this is easier said than done and it’s probably one of the toughest challenges for entrepreneurs. Hiring without capital is tough. Raising capital without a team is even tougher.

This challenge becomes slightly less hard for serial entrepreneurs with a good track record for two reasons.

Firstly it’s easier to raise pre-seed rounds without having a team in place. In Rnwl’s case, investors bet on my ability to execute and hoped for the best.

Secondly, potential hires also take a bet on the entrepreneur‘s track record and vision. This meant potential hires were open to joining an unproven startup by balancing the risk of never getting anywhere versus the opportunity of being involved with a great company from the very beginning. The highly experienced team behind Rnwl is making a huge difference in terms of speed and quality of execution.

So my advice to you is to hire the very best you can afford. Think outside of pure salary: option schemes, the excitement of being involved in a startup, the ability to define the roadmap of the company and the working culture are all points in favor of a good startup when compared to large corporates. Use them to your advantage.

3) Obsess about customers, product and the value proposition, not the fund raising

Raising capital is a worthwhile exercise as entrepreneurs are able to capture valuable feedback from potential investors, even if they don’t invest. However, spending too much time fund raising is going to divert the entrepreneur‘s energy and focus from what really matters: the customers.

When I founded SR I had to spend far too much time trying to raise capital. This meant that I didn’t have enough time to focus on the customers, the product and the value proposition. Raising capital is always time consuming, but with Rnwl I was able to strike a much better balance. I was able to focus far more in understanding what customers valued in the product. As a result we were able to nail the value proposition behind Rnwl, even if we are not disclosing it quite yet.

The reality is that if you’ve nailed your value proposition you will have a much easier time with potential investors. Get the value proposition right. Talk to potential customers and find out what they really think and keep your mind open to tweaks and changes to your product and idea.

4) Do everything faster

You can save a lot of time by using online tools that make your life much easier. Xero or similar (plenty of options around) for accounting, SeedLegals or similar for capital raising, Vestd or SeedLegals for options’ plans, Expensify for employee expenses, ReceiptBank for receipts, one of the challenger banks for company cards and so on.

Anything that allows you to get on with things and focus on your customers, your product and your value proposition is worth the usually small monthly fee. Use these time saving tools and move on. Focus on what adds value to the company: the customers.

5) Focus

Entrepreneurs have a tendency to be attracted by the unknown and the shiny new opportunity for the business to expand. I’m very guilty of this. However, this is rather counterproductive when building a startup. Serial entrepreneurs are far better at focusing on the core vision and product. They define what the value proposition is and then stick to it. They make sure they deliver and leave satellite opportunities alone. It’s powerful to say ‘No’.

It took me far too long to learn this last one and, I hate to admit, I still struggle with it. I mean, how am I supposed to resist the latest shiny new opportunity? Except this time I did and the team behind Rnwl is laser focused on one common goal. The difference is noticeable. After all, there are good reasons why it was so much quicker to get to where we are this time around.