Complete hiring guide for JavaScript developers

JavaScript has always been popular in the IT industry because it allows developers to quickly and easily make websites that resemble apps, all with the help of one technology set and not too much hassle or time spent.

It is not the most straightforward task to choose a candidate out of many that applied.

Before continuing with the hiring process, you initially need to determine the type of JavaScript developer you need for your project. We will cover this, interviewing questions, and tips to avoid challenges and obstacles during the process—technical and non-technical skills, screening phases of the interview, and much more.

We also include helpful information directly from recruiters and developers, benefits and stats about JavaScript, and everything valuable to you to shorten the hiring period without sacrificing quality.

About JavaScript

JavaScript is an object-oriented, lightweight, dynamic programming language (scripting language) commonly used for creating web apps, web browser interactive effects, game development, and more.

Previously known as LiveScript, this programming language was first released back in 1995 as an integrated part within Netscape Navigator 2.0 (web browser), but later named LiveScript.

Despite being a programming language for web page use, JavaScript can also be perfectly intended and used for non-browser environments and uses. This dynamic language offers support for object construction based on prototypes, and its syntax resembles both Java and C++.

Stats about JavaScript

The salary of a JavaScript developer is, on average, $94,260 annually, with the highest pay being $109,796—a full stack position starts around $109K and above annually. Despite this fact, JavaScript is ranked lower than the most paid technologies.

The good thing about this programming language is that it is in high demand, but this fact does not correspond to the experts available. 75% of companies and employers actively need to find and hire a JavaScript developer.

The popularity of this language is outstanding, with 68.62% of surveyed developers preferring it the most for their work. This fact secures JS with first place on the popularity rank list, and a smaller percentage of 14.59% claimed they want to use JavaScript or implement it in their work.

Popular brand names that rely on JavaScript usage

JavaScript is a popular language with the following brands/companies:

1. Microsoft
2. Uber
3. Netflix
4. PayPal
5. Groupon
6. Google
7. Facebook
8. LinkedIn
9. eBay
10. Walmart
11. IMDB
12. Twitter
13. Trello
14. BBC

Interviewing a JavaScript developer

Helpful tips for a pre-screening interview

A critical segment of the whole finding and hiring process is to have a talent acquisition specialist conduct the pre-screening of the developers. These specialists have specific questions and assessment tasks that help evaluate a candidate's expertise.

Recruiters and talent acquisition specialists always inquire about past work experiences, in terms of years of working with JavaScript because this helps in determining the seniority level of the developer.

Another thing they check is the linguistic aspect or proficiency in English to check if there will be potential challenges in the daily communication between the client and developer.

Also, soft skills are crucial too, such as communicativeness, punctuality of the developer for the call, and the environment around the developer. Professional experience combined with appropriate appearance and attitude always bode well.

Technical skills of a JavaScript developer

The experienced JavaScript developer should have the following skills or extensive knowledge:

• JavaScript
• CSS
• HTML
• jQuery (JavaScript framework)
• Asynchronous programming (Prototypes, Scope, Hosting, Coercion, etc.)
• Cross-browser code writing
• React JS (library)
• Node JS (tool)
• TypeScript

For this, we also asked Diamant Isufi, a software engineer in Proxify, to describe more about what else does the developer need to know, and he said:

“Besides the problem-solving skills, the developer must master all JS basics. Starting from how it works, when it is used and why, but also accurately describing simple data types, variables, functions, classes, constructor functions, and modularity.”

Diamant Isufi

Non-technical skills of a JavaScript developer

The non-technical skills are a big chunk of the overall interview impression.

The recruiters know that not everything is solely expertise, technical knowledge, and experience. The essential minimum expectation is, of course, English language proficiency and being on time for the interview. Punctuality is a good indicator of the level of responsibility and respect the developer has for the recruiter, company, or client.

But, it doesn’t stop here. It is good, even strongly recommended, that the developer attends the call in a suitable environment/space. The environment needs to be neat, with enough lighting, good quality camera, and a headset (for a clear sound) if possible. The candidate should follow an appropriate dress code and be in a room without loudness or distractions.

Another thing for recruiters to focus on is the conversational skills of the candidate or how they handle the conversation flow. Listening without interruptions, talking clearly, or politely asking to repeat a question are plus points for the developer and add ‘points’ to their communicativeness aspect.

Essentials and assessment of a JavaScript developer

We mentioned above the skills and technical knowledge of a JavaScript developer; however, it’s good to sum up the overall essentials to look for:

• Expertise in JavaScript, HTML, CSS and jquery
• Knowledge of React JS, Node JS, and TypeScript
• Knowledge of React and Redux
• Good cross-browser coding skills
• Asynchronous programming

But, also:

• An analytical mindset that keeps up with fast-paced novelties
• Knowledge of how various browsers or platforms function
• Knowledge of Meteor (JS platform)
• Version control experience
• ES6 language features-(ECMAScript 6-for standardizing JavaScript)

Diamant here further added:

“The developer also needs to know about ES7 (exponentiation generator). In general, I would also assess the problem-solving skills and ask about the frameworks and libraries of JS. After this, I would ask about their experience with Angular, Vue, and React. But, also overall, the technologies they need to know well are DOM (document object model), Node.js, NPM and Yarn, and NVM (node version manager), too.”

What distinguishes a great JavaScript developer from a good one?

There is always a fine line between excellent and (very) good developers, but once you see something that makes a difference, it can be crucial. In this case, the experienced and ideal JavaScript developer candidate must have substantial knowledge and experience with this programming language.

Also, they need to be able to work on desktop apps, hybrid mobile apps, games development, backend web apps, microservices, network services, as well as embedded apps with specific tools.

Additionally, a great developer candidate needs to know about ECMAScript frameworks and versions and use the best IDE (integrated development environment).

Possible challenges during the hiring of a JavaScript developer

There are challenges and certain obstacles during hiring processes, but it is important to recognize or anticipate them to avoid or resolve them as they appear.

One possible challenge is a scarcity of experts that genuinely have extensive in-depth knowledge and fit the criteria and requirements of the job position. Then further, there is the challenge of miscommunication regarding salaries or financial expectations that would correspond to the volume and complexity of the obligations.

In this context, if the demand for JavaScript devs on the market is high, there will always be competitors trying to ‘get’ the best developers for themselves. With this in mind, the whole recruiting process becomes more urgent and incredibly dynamic.

And of course, the recruitment process is costly by itself, so ideally, you’d need it to result in the best dev ‘ending up’ working for your company.

Why and when do you need to hire a JavaScript developer?

You can expect improvements in your business through the following aspects once you decide to hire a JavaScript dev:

• ‘Bang for a buck’—JS is a fundamental programming language, and if the dev knows it, they probably also master other languages and technologies too.
• Creating sable web servers and apps
• Dynamically updated content
• Easy control of videos, images, and multimedia elements
• Emphasizing easily through animation
• Game creation and development
• Utilizing powerful frameworks—library of many structures for project particulars, with numerous features for quick and good quality development
• Fullstack development—both frontend and backend are written easily
• Simplicity of debugging and testing—identifying bugs quickly, codes are reviewed simple line by line
• Creating cross-platform mobile and web apps
• Excellent interfaces—creating captivating and visually gorgeous interfaces for the users, with drag and drop and more interesting and valuable features.
• Concurrency—running more tasks at the same time, handling different operations, thus saving time overall
• Cybersecurity

Diamant sums up the beneficial aspects of JS as:

“A JavaScript dev will be able to write codes for multiple platforms, and they are likely highly flexible to adapt to new frameworks and libraries that use JS, such as Angular, React, and Vue. Writing mobile and desktop apps is another notable benefit. Expertise in JavaScript is beneficial when a business is scaling, and demand for other platforms is raising."

Benefits of JavaScript

• Popularity—JavaScript is a popular programming language with many uses and implementations. As mentioned, a skilled dev can do related and other tasks not strictly confined to just JS.
• Speed—JavaScript allows for fast work and development within the client-side browser, often without the need for external resources
• Simplicity—Easy to learn and implement in the work
• Creation of excellent interfaces
• Multiple actions—JavaScript matches nicely with other programming languages and offers quality usage in other apps.
• Open source projects access—the developer that needs to find assistance or guidance can easily do so
• Simple syntax
• Versatility
• Not depending on a platform
• Reduced server interactions

For the overall benefits and perks of working with JavaScript, Diamant also commented:

“I like this language's speed since it is not a compiling language but an interpreted one. It is prevalent on the web among all devs, so a massive community of experts offers quality information and access to needed resources, articles, courses, and more. And, the server-side rendering is another major benefit with JavaScript.”

JavaScript interview questions and answers

Many questions are relevant for testing the expertise of the JS developer, but here are some suggestions that you could use during the interview:

1. What is isNaN, and how is it used?

Expected answer: The phrase ‘NaN’ means ‘Not-a-Number’, so subsequently, isNaN lets us know if some value is NaN. More precisely, we use the function isNaN( ) if we need to discover whether some value holds an illegal or incorrect number. So, if this function/method comes back as ‘true,’ it means the value we examine is indeed NaN.

2. Explain what is ‘negative infinity.’

Expected answer: When we divide a negative number by zero, we get a Negative Infinity number in JS. This negative infinity number is a constant value that is the lowest available value we can receive, and there will be no lower number than this one.

3. What are ‘global variables’?

Expected answer: The global variables are variables that are visible from all scopes and declared in a global scope—they are always declared through a window object or outside of a certain function, and they are accessible from literally any function too.

In simplified terms, a global variable will keep its own value as long as the program exists through its entire lifetime.

4. Explain the difference between SessionState and ViewState.

Expected answer: The SessionState is data that continuously persists in a specific server, and the data availability is stopped when the browser is closed or the session is expired. It is super-specific data with access from all pages of the web app. The ViewState is a group of controls’ values on a certain page on the client browser side. While SessionState is saved on a server, the ViewState is saved in the page itself, and it can be seen from one page, not multiple ones. Basically, ViewState will store the info only on the client’s side.

5. What are ‘looping structures’?

Expected answer: When we use a loop or looping structures in JavaScript, we use them for some tasks that need repetition, i.e., this feature helps us to repeat functions. The loops do not stop running unless their condition appears as ‘false.’ We got a total of 7 looping structures in JS, listed below:

• do-while
• while
• forEach()
• for
• for…in
• for…of
• map()

6. Explain the meaning of ‘NULL’ in JS.

Expected answer: The meaning of ‘null’ is no object and/or no value. This is also known as ‘null string’, and it is valueless intentionally. We also call this a primitive value, and it has no array object and an incorrect boolean value too.

7. How is ‘Void (0)’ used?

Expected answer: Simply put, a void is something that is empty; of course, it has nothing within. In JS, ‘void’ equals ‘return of nothing’ or an empty value. So, with void(0), we are calling another, different method without page refreshing. With the help of ‘void(0)’, we prevent any obstacles and side effects to a page when we need to insert something on the page, like an expression.

8. What is the ‘pop()’ method?

Expected answer: When I need to remove the last array element, I use the ‘pop()’ to retrieve that same element. With this, I adjust the array length accordingly, and I do this by this syntax: array.pop( )

9. State the difference between ‘.call()’ and ‘.apply()’.

Expected answer: These two are a bit similar, but let’s break it down. When I know the arguments of a function and its number, I use .call(), and this function is a predefined method. This method allows a specific object to take on a method from a completely different object. When I use .apply(), I use it if I have no clue what the number is, but I could write a method to use on more, various objects.

10. How do ‘deferred scripts’ in JS work?

Expected answer: First, we need to define the ‘defer’ in JS, which is a boolean attribute, and its role is to determine when the downloaded script is in synch and parallel to the page parsing. It’s essential that the script is executed when the parsing is completed for that page. Now, with deferred scripts in JS, scripts postpone script execution when the HTML parser is active. The outcome of this is quick displaying and less web page loading time.

11. What are arrow functions in JS?

Expected answer: With the help of arrow functions in JS, we can create anonymous expressions or write the expressions for anonymous functions. These arrow functions do not offer support for keywords like ‘new. target’, ‘arguments,’ ‘this,’ or ‘super’. There is quite a vivid similarity between JS arrow functions and the Python lambda functions. In a nutshell, arrow functions are used when we need to make a function pass just like a parameter for some more demanding function.

12. List all the JavaScript Data types.

Expected answer: The JavaScript Data types are 6 in total, but they are divided into three categories.

• Primary or primitive—These are the Boolean, String, and Number.
• Composite (or reference)—These are the Object, Function, and Array
• Special—These are the Null and Undefined data types.

13. Differentiate between undefined and not defined variables.

Expected answer: Both types of variables are closely linked to memory space. Simply put, an undefined variable is the one that exists in the memory space but has no value assigned to it up to now. The not defined variable is the non-existent variable in the memory space, but it’s still referenced somewhere.

14. What are JavaScript timers?

Expected answer: When we need to execute a specific function or task within a certain timeframe, we use timers. The timer can help us either delay the execution of a program or code or help execute the code on time. All timers operate in one single thread, so subsequent events and functions will queue up afterward.

15. Elaborate on automatic type conversion and JavaScript.

Expected answer: Automatic type conversion is a process of value assigning. More precisely, it is a process where we assign a certain value from one numeric data type to a whollye different numeric data type—in JavaScript, this sort of automatic type conversion is enabled and supported and frequently used.

16. What is the event delegation model in JavaScript?

Expected answer: With the JS delegation model, we can achieve better organization and events handling, and this is one of the best features of JavaScript. When we want to use one handler for several elements simultaneously, we bubble and then capture the mentioned handler to apply it to more elements and parts simultaneously—this is the event delegation that simplifies the work immensely.

17. Define JavaScript cookies.

Expected answer: When a user visits a website, the website stores tiny test files in the computer. These small test files are the cookies, and they are created whenever the user clicks on the website to input some information. This can be personal information during checkout for a shopping cart or filling out forms, and similar, after the website asks for permission of this kind and the user types it in.

18. Can you briefly define the escape characters?

Expected answer: When devs need to execute a specific operation through an escape command, they need to type a symbol (a backslash, “ \ ” ) to start that escape command. That symbol is called an escape character, and it is used mainly with specific characters like ampersands, single quotes, double quotes, apostrophes, and similar. The backslash will become visible only if you type it before the characters.

19. Explain the JavaScript generator.

Expected answer: A generator represents a function or process that can be re-entered once we exit from it and can also be paused before we continue working with them. The good thing about using the JS generator is that midway, you can pause or stop and come back to continue where you left off.

20. How many error types are there in JavaScript, and define them.

Expected answer: With JS, there are three types of errors, Syntax, Runtime, and Logical.

The Syntax errors are also known as parsing errors—these happen if we want to interpret a code, but that code is invalid syntax-wise. Syntax errors are common and frequent. For example, if you type a semicolon, but instead, you should’ve typed a double-colon, this will be a syntax error.

The Runtime errors are also named and known as exceptions. These happen mostly right after the compilation and interpretation, and they occur when during the runtime, we call a function that is not declared, existing, or present at the time. In this case, the syntax has no mistakes.

The Logical errors occur when there is an issue with the code, but otherwise, the syntax is correct and valid. In this case, we might run the program smoothly, but something appears wrong in the end with the results. This error type is quite complex to resolve and identify and is not linked to runtime or syntax. Logical errors occur with faulty data flow or if the dev makes some mistake in the script logic.