Top 5 Interview Questions Every JavaScript Developer Should Know

Hello IT world! JavaScript developer’s position looks tempting not only because of the size of the salary, but also because of the great demand for this position among employers, because this type of profession has been in the top of the most demanded specialties in the IT field for a long time. If you are reading this article which is called ‘Top 5 Interview Questions Every JavaScript Developer Should Know’, you probably have the idea of joining the ranks of JavaScript developers. If you already had surfed information on the Internet, watched videos about what a JS developer should be able to do and where to start studying, or monitored a list of requirements for vacancies, your hair could stand on end from the list of all these necessary skills, even for a junior position. It may seem to you that the stack of necessary knowledge and skills of such specialists is so large that it will be unrealistic to pass a successful interview. Therefore, I have compiled for you a list of top 5 questions that you should know the answer to in any interview. Read on and discover!

At the stage of learning JavaScript, many novice web developers make a big mistake, neglecting the basics of JavaScript and jumping straight to frameworks. But I want to note that there are basics that you need to know. These include: basic understanding of the language and its features; data structures; strings, numbers, objects and arrays (you need to know the methods to add a value to an array, turn the word backwards); closure and scopes (how it works and what it means); object methods (Bind, Call and Apply); execution context and function call stack (you should understand the difference well and be able to use it by examples). Also, employers pay attention to understanding how the GIT version control system works. Working in a team, you should be able to control your code, for this knowledge of 3-5 commands will be enough. It is also important not only to read the information, but to immediately apply all knowledge in practice. The more practice you will have, the faster you will get results.

Now I propose to move on to the top 5 questions, which should be known when passing an interview for a JavaScript developer position. Fortunately, you can prepare for the technical part of the interview and be sure not to fail. Of course, interviews are different: you may or may not be asked questions of this kind, but there is no unnecessary knowledge 😉

  1. What is the difference between inheritance in JavaScript and inheritance in PHP? Unlike PHP, where inheritance could be done in one way, JavaScript has many such inheritances. At the language level, inheritance on prototypes is implemented. In JavaScript, each object can have an association with another object – the so-called prototype. If the search for some property (or method) in the original object fails, the interpreter tries to find the property (method) of the same name in its prototype, then in the prototype’s prototype, and so on. For example, if we requested a call to obj.prop, JavaScript will start looking for the prop property in the object itself, then in the obj prototype, and so on.

  2. What are global variables? How are they created and what problems are associated with them? A global variable is a variable that is available (visible) throughout the document, as opposed to a local one (it is limited by the framework of the block of program code within which it is defined). Most JavaScript developers avoid using global variables. One of the reasons is a possible conflict in the names of global and local variables. Also, code that uses global variables can be harder to maintain and test. Therefore, such a question from the basics of JavaScript is an indicator of whether the developer can cope with possible difficulties in writing code or not.

  3. What is a JavaScript closure? In simple words, this is an internal function, that is, a function that is located in another function. It is called a closure because after the execution of the parent or outer function, the inner code still “lives” in the interpreter and can be executed. No errors occur even if the closure uses variables from an external function. This feature can be used to create event handler functions.

  4. What will the expression + new Date() return and how is it different from
    The answer is that + new Date() will create an instance of the Date object and, thanks to +, will convert it to a number format. In the second case, a static constructor method is called, which has a higher priority, since, firstly, it does not require creating an instance, and secondly, it is more understandable.

  5. What is the best method for creating a DOM element? When you set the innerHTML property, the browser removes all children from the element. Then it parses the string and inserts it into the element as a child. For example, if you want to add a list item to an unsorted list, you can get the item and set the innerHTML property to it. But innerHTML can be slow when parsing a string. The browser is forced to deal with the string even if you gave it an invalid html. On the other hand, while you are using appendChild you are creating a new element. Since its creation, the browser doesn’t need to parse the string and deal with invalid html. And you can specify a child for the parent to be added to the parent element. Still, it is better to write a couple of extra lines in JavaScript because it will simplify the task of the browser and make your web page faster.

These were top 5 questions, that in my opinion, require special attention. In addition, every JavaScript developer should answer them without hesitation. But it is worth noting that there are more than a dozen questions that are very popular in such interviews. Therefore, do not be lazy and flip through the lists of questions on different IT forums and try to highlight some for yourself.


A technical interview for a JavaScript developer position in 99% of cases starts with general questions, and then moves on to more tricky ones that require attention when studying. Even if you made a mistake with some answer, then do not worry, because the train of your thoughts will let the employer understand that you have an understanding of what is required of you. Show in the interview that you are passionate about JavaScript, that you are constantly learning and want to know more. Technologies in IT are developing at a rapid pace and you must be ready not only to constantly learn new things, but also to monitor the updating of existing knowledge. It is very important to learn how to filter information and not be scattered over many articles “about everything and nothing.” Self-study can take more than a year, while optimized courses will help you master the front-end direction in 5-7 months. Thanks for reading the article ‘Top 5 Interview Questions Every JavaScript Developer Should Know’. Hope it was interesting and useful for preparation for the interview!