An interview is an important component of the recruitment process. It provides insights into an individual’s skills, knowledge, and ability to take on challenging tasks. It is also one of the most reliable ways for employers to screen candidates who are not suitable for the position.
What are some common JavaScript interview questions? As a JavaScript developer, you must be prepared before taking an exam or interview. This will increase your chances of being hired by a reputable company.
Now you might be wondering where to start preparing or what questions they will ask in an interview.
So, to help you out, I’ve put together a list of common JavaScript interview questions collection. Each of these questions has a short answer, which you can check out below.
In the end, you will be more confident in answering any questions from your interviewer. So, without further ado, let’s get started.
What is JavaScript?
JavaScript Interview Questions Explained: JavaScript is a scripting language created by Netscape in 1995. It was originally used in the popular web browser, Netscape Navigator. However, nowadays, we can use it for both client-side and server-side application development.
Well…… I don’t think anyone would ask this question, but we shouldn’t forget where JavaScript came from.
List some of the advantages of JavaScript?
- JavaScript can work offline in a web browser.
- It supports a wide range of programming paradigms. For example, we can use object-oriented, functional, and imperative programming concepts.
- It has the largest collection of open-source libraries and frameworks.
- JavaScript enables the creation of online/offline games, desktop software, websites, and mobile applications.
- There’s no need to learn a separate programming language to create the front-end and back-end of your website. JavaScript is supported by all major web browsers, and it can run on servers that use Node.js.
- It is an interpreted language. This means that we don’t have to build or compile its code before using it. JavaScript directives are executed directly.
Should we use internal or external JavaScript?
What are some common JavaScript interview questions? Internal JavaScript is more appropriate when we only need to use it on a single web page. For websites with multiple web pages, it is always preferred to use an external JavaScript file.
Why do WordPress plugins like Autooptimize aggregate JavaScript code in a single file?
The aggregation of JavaScript source code in a single file reduces the number of requests made to the server when a web page is generated. In turn, it makes the website load faster.
For example, let’s say we have 10 JavaScript files on a web page. Now, when we open this web page, our web browser sends 10 HTTP requests to the server to retrieve these files. On the other hand, if we aggregate the code of all these files into one, then we only need to make 1 request to the server.
What is JavaScript Boost?
JavaScript Interview Questions Collection: Ascension is a concept in JavaScript that allows us to use variables and functions even before they are declared.
Basically, when we execute a piece of JavaScript code, it first automatically extracts all the variable and function declarations from the code and moves them to the top of their scope. After that, it starts executing the code.
The main benefit of the boost is that our code works fine and doesn’t show any errors such as “undefined variables” or “undefined functions”.
Learn more about boosting in my article Understanding Variables, Scopes, and Boosting in JavaScript
Predict the output of the following code?
Code:-
username = "Juan Cruz Martinez";
var username; // variable declaration
var username; // redeclared the variable
console.log(username);
Output:-
Juan Cruz Martinez
Interpretation:-
- First, the variable declarations on lines 2 and 3 will be treated as a statement.
- Now, the concept of hoisting will be applied. This means that JavaScript will move variable declarations to the top. After that, the code will be executed.
- Also, keep in mind that even if we redeclare a variable, the value stored in the variable is not lost.
What is JavaScript “Strict Mode”?
What are some common JavaScript interview questions? JavaScript’s default behavior is very forgiving in case we make small mistakes. This means that it won’t show any error messages. However, sometimes in development, we need to look at various errors and warnings to debug the code.
Here’s the use of “strict mode” in JavaScript. Basically, it’s a restricted variant in which JavaScript displays all errors and warnings, even if they’re silent.
We can enable “strict mode” using the directive at the beginning of the script."use strict";
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
- Horn
What is a self-invoking function?
Self-calling functions are slightly different from normal functions in that they are executed immediately where they are declared.
Typically, we declare a function first and then call it. However, JavaScript automatically executes code that calls functions at runtime.
One thing to note is that these functions don’t have any names. In turn, we cannot recall these types of features. They are also known as “anonymous functions”.
This is an example of a self-invoking function.
(function (){
var a = 12;
var b = 3;
console.log(a * b);
}());
What is the difference between “var”, “let”, and “const”?
| Reactive | let | constant |
|---|---|---|
| It has been available since the beginning of JavaScript. | This is a new way to declare variables in JavaScript, starting with ES6. | const is used to store values that don’t change throughout the execution of the script. It was also recently introduced in ES6. |
| It has a global/function scope. | It has a block scope. | It also has a block scope. |
| It can be updated or restated within its scope. | We can’t re-declare them. | const represents a constant value, so it cannot be updated or redeclared. |
Learn more about them in my article Understanding Variables, Scopes, and Boosts in JavaScript
What is the difference between “==” and “===”?
They are both used in JavaScript to perform comparisons between two values.
| == operator | === operator |
|---|---|
| It is a type conversion operator. | It is a strictly equality operator. |
| It is only used to compare two values. | It compares two values and their types. |
Is there any difference between the “null” and “undefined” keywords?
JavaScript Interview Questions Collection: Both of these keywords represent a null value. However, there are two basic differences between null and undefined.
| null | Unambiguous |
|---|---|
We declare a variable and assign a value to it.null | We declare a variable without assigning a value. |
| (empty) The type of the object | (Undefined) The type of “undefined”. |
What is the difference between “function declarations” and “function expressions”?
JavaScript Interview Questions Explained: Basically, a function declaration is nothing more than the normal process of defining a function using keywords, its unique name, parameters, and function body. And when we assign a function declaration to a variable, it becomes a Function Expression.function
Interestingly, the function expression is not promoted. This means that if you try to call them before defining them, they will show an error.
Function Declaration:-
function full_name(first_name, last_name)
{
return first_name + " " + last_name;
}
Function Expression:-
var addition = function add(value_1, value_2)
{
return value_1 + value_2;
}
What is a closure
What are some common JavaScript interview questions? A closure in JavaScript is a function within another function. An inner function can access its own variables, variables defined in the external function, and global variables.
Closing example:-
var a = 2;
function outer()
{
var b = 4;
function inner()
{
var c = 6;
console.log(a + b + c);
}
inner();
}
outer();
Predict the output of these two functions? Will they return the same output?
function user1()
{
return {
name: "Juan"
};
}
function user2()
{
return
{
name: "Juan"
};
}
console.log(user1());
console.log(user2());
Output of function user1().
{name: "Juan"}
Output of the function user2().
undefined
What is NaN?
In JavaScript, NaN stands for “non-digital”. This is a special value that appears when we are unable to perform an operation.
For example, what if we try to split a string with a number such as “Hello World” / 5.
Explain the for-in loop?
JavaScript Interview Questions Collection: for-in loops are specifically designed to loop through all the properties of an object step by step. It selects a property from the object in each iteration and performs the desired action on it.
Let’s try to understand it with the help of an example.
var user = {
"name": "Juan",
"country": "Germany",
"website": "livecodestream.dev"
};
for(var key in user)
{
if(user.hasOwnProperty(key))
{
console.log(key + " -> " + user[key]);
}
}
Output:-
name -> Juan
country -> Germany
website -> livecodestream.dev
What is Event Bubbling and Capture?
In the JavaScript DOM, HTML elements are nested within each other to form a hierarchy.
Now, if both the parent and child elements have registered handles for a particular event, what is the order in which the events are propagated?
It can be determined in two ways, known as event bubbling and capture.
In an event bubble, the child element will first capture the event and then propagate it to the parent. In event capture, the parent element captures the event first and then propagates it to the child element.
What is the difference between JavaScript and ECMA scripts?
JavaScript Interview Questions Explained: JavaScript is a scripting language, whereas ECMA Script is a set of guidelines and rules that are used to standardize JavaScript in different web browsers.
How do I create a cookie using JavaScript?
What are some common JavaScript interview questions? In JavaScript, you can create a cookie using the document.cookie object. Simply assign it a string value, which is simply a series of key-value pairs separated by semicolons.
JavaScript creates cookies for example:-
document.cookie = "username=Juan; expires=Wed, 03 Oct 2030 12:00:00 UTC; path=/";
JavaScript Interview Questions Collection Conclusion
Employers often try to confuse applicants by asking tough questions. Therefore, if you are not fully prepared, then you will most likely end up losing your chance.
So, today, I’m trying to answer some common JavaScript interview questions. You can even use it as a reference before the interview.
Thanks for reading!
