JavaScript Interview Questions

If you are looking for a position as a JavaScript programmer, it is pretty sure that you will be facing some interview questions and challenges. We have selected for you the most commonly asked JavaScript interview questions and their respective answers, at both basic and advanced levels. 

Furthermore, we had included several JavaScript Code Challenges that will help you to be more prepared for your interview ahead. So we hope that when you mastered our selection, you will be ready to excel in your JavaScript interview. 

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Basic JavaScript questions

What is JavaScript?

JavaScript is described as a lightweight, dynamic, interpreted programming language. It has object-oriented capabilities, so we can use it to interact with otherwise static HTML pages. The JavaScript core/code has been embedded in almost every web browser, so it is universally used.

Which are the JavaScript features?

JavaScript is a lightweight, interpreted programming language.

JavaScript is designed for creating network-centric applications.

JavaScript is complementary to and integrated with Java.

JavaScript is an open and cross-platform scripting language.

Mention some advantages of JavaScript

Less server interaction: We can validate user input before sending the page. That reduces server traffic and load.

Immediate feedback: users don’t have to wait to see if they have entered data incorrectly.

More interactivity: JavaScript allows creating interfaces that react to the user’s actions, like hovering over with a mouse or using the keyboard.

Richer interfaces: we can include items like drag-and-drop components or sliders to improve the user experience.

Are Java & JavaScript different programming languages? 

Yes, they are. While Java is an OOP programming language, JavaScript is an OOP scripting language, so it creates applications that only run in a browser. Another difference is that Java code needs to be compiled, while JavaScript code is only text.

What data types are supported in that JavaScript?

Data types supported by JavaScript are:

BigInt: used to store numbers which are above the limitation of the Number data type. Is represented by adding “n” to an integer literal.

Boolean: it represents a logical entity (true or false), used for conditional testing.

Null: It represents an invalid or non-existent value.

Number: it represents a number, with or without decimals.

Object: it is used to store a collection of data.

String: it represents a series of characters. A string can be represented using a single (') or a double (“) quote. 

Symbol: it is used to store an anonymous and unique value.

Undefined: it represents a variable that has been declared but not assigned.

Is JavaScript case-sensitive?

Yes, it is a case-sensitive language.  The language keywords, variables, function names, and any other identifiers must always be typed taking capitalization into account.

What does the ‘This’ operator do?

In JavaScript, the ‘this’ keyword refers to the object it belongs to. It has different values that depend on where it is used. In a method, it refers to the owner object. In a function, it refers to the global object.

Explain what a Callback is

A callback is a plain JavaScript function given to some method as an option or argument. Basically, it is a function that will be executed when another function ends. It must be remembered that in JavaScript functions are objects. So functions can take functions as arguments, as well as be returned by other functions.

What is a Closure in JavaScript?

When a variable defined outside the current scope is accessed from within some inner scope, a closure is created. It gives us access to an outer function’s scope from an inner function. 

Closures are created every time a function is defined. So, when we define a function inside another function and expose it, a closure appears.

In JavaScript, what is the scope of a variable? 

The scope is where in the program the variable is defined. In JavaScript variables can be:

• Global: then, the variable is visible everywhere in the code.

• Local: the variable will be visible within a function where it is defined.

Mention the variable naming conventions in JavaScript

When naming variables in JavaScript, we should not use any reserved keyword such as break or boolean. Variable names should not start with numbers, but with a letter or the underscore character. Also, it must be remembered that in JavaScript, variable names are case-sensitive. 

How to read a cookie with JavaScript?

In JavaScript, the value of the document.cookie object is the cookie. It is a string with a list of name and value pairs, separated by semicolons (name is the name of the cookie and value is its string value).

Therefore, we can use that string whenever we need to access the cookie.  For instance, we can use the split() function to separate the string into key and values.

Mention some of the built-in methods in JavaScript 

reverse(): reverses the order of the elements of an array.

push():adds one or more elements to the end of an array and returns the new length of the array.

pop():removes the last element from an array and returns that element.

length(): returns the length of the string.

indexOf(): returns the index within the calling string object of the first occurrence of the specified value.

forEach(): calls a function for each element in the array.

Concat(): joins two or more strings.

CharAt(): returns the character at the specified index.

Is JavaScript a statically or a dynamically typed language?

In a dynamically typed language such as JavaScript, the type of a variable is checked during run-time, not in the compile-time process. JavaScript is a loosely typed language, where variables are not associated with any data type. 

Advanced JavaScript interview questions 

How an HTML element can be accessed in a JavaScript code?

An HTML element can be accessed in a JavaScript code by:

getElementById(‘idname’): Returns an element by its ID name

getElementsByClass(‘classname’): Returns all the elements from a given class name.

getElementsByTagName(‘tagname’): Returns all the elements included in the tag name.

querySelector(): it takes a CSS style selector and gives the first selected element.

How a JavaScript code can be involved in an HTML file?

JavaScript code can be involved in an HTML file in three ways: Inline, Internal or External.

The inline function is a JavaScript function assigned to a variable created at runtime and can be easily reused. Another way is to integrate the script into the page (internal) or put it into a different file that we may call when needed (external).

How we can define a variable in JavaScript?

The three ways of defining a variable in JavaScript:

Var:  With the JavaScript variables statement, we declare a variable. We can also initialize its value (For instance: var a =10). Declarations are processed before the execution of the script.

Const: the const function does not allow them to modify the object. When the function is declared const, it can be called on any type of object.

Let: it indicates that the variable may be reassigned (like in a counter in a loop), and also let us know that the variable will be used in the block it’s defined in.

Explain the difference between null and undefined

If a variable has been declared but has not yet been assigned a value, it is undefined. Meanwhile, null is an assignment value as a representation of no value. Undefined and null are also two distinct types: undefined is a type, while null is an object.

Explain the difference between undeclared and undefined

If a variable has not been declared in a program, it is undeclared. So, if we try to read the value of an undeclared variable, a runtime error appears. 

On the other hand, a variable is undefined when we had declared, but no value had been given. So, when the program read an undefined variable, its value is also undefined.

Explain the difference between local and session storage

In local storage, data is not sent back to the server for every HTTP request, reducing the traffic between client and server. Data will remain until cleared, through settings or by a program. 

Session storage is similar, but the data stored in the session gets cleared when the page session ends (when the browser is closed, for example).

What is a JavaScript Framework?

JavaScript Frameworks are application frameworks written in JavaScript, not JavaScript libraries, so it control flow is different. Some of the most commonly used JavaScript frameworks are Angular, React, and Vue.

What is “event bubbling” in JavaScript?

Event bubbling is when an event occurs in an element inside another element, and both elements have a handle for that event. It is a way of event propagation in the HTML DOM API: the event is first detected and handled by the innermost element and then passed to outer elements. 

How do we convert the string of any base to an integer in JavaScript?

We should use the parseInt() function. It takes as the first parameter the string to be converted, and as the second the base of the given string.

For instance, 

1 parseInt("4F", 16)

What is the ‘Strict’ mode in JavaScript?

Strict mode helps to do better error-checking in our code since in that mode we cannot use implicitly declared variables, add a property to an object that is not extensible, or assign a value to a read-only property.

To enable strict mode, we just have to write “use strict” at the beginning of the file, program, or function.

What is a prompt box in JavaScript?

Is a box that allows the user to enter data, provided in JavaScript by the prompt() method displays a dialog box that prompts the visitor for input. It is often used if we want the user to pass values to the script before entering a page. This prompt box is a pop-up to click “OK” or “Cancel” to proceed, after entering an input value.

Why it is a common practice to wrap the content of a JavaScript source file in a function book?

Many popular JavaScript libraries do that nowadays. Making a function book creates a closure around the contents of the file which creates a private namespace. Thereby, it helps avoiding name clashes between other JavaScript modules and libraries. The function book also allows creating an alias for a global variable, a procedure often used in jQuery plugins.

Explain what are escape characters in JavaScript

Escape characters (backslash) in JavaScript allows us to write special characters like single quotes, double quotes, apostrophes, and ampersands when placed before the characters to display.

Explain the use of promises in JavaScript

Promises have been implemented in JavaScript to handle asynchronous operations in a more efficient way than callbacks. A promise object has four possible states:

Pending: is the initial state of promise, where the promise has neither been fulfilled nor been rejected.

Fulfilled: when the promise has been fulfilled and the asynchronous operation has been completed.

Rejected: the promise has been rejected for some reason and the asynchronous operation has failed.

Settled: the promise has been either rejected or fulfilled.

JavaScript Code Challenges 

So, with these challenges, we have reached the end of this article on JavaScript Interview Questions. We hope that you feel now more confident and remember: make sure you study and practice as much as possible. Good luck with that interview ahead!

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Basics: What is JavaScript

JavaScript is considered as one of the most popular languages to develop dynamic web pages. Created in 1995 by Brendan Eich, JavaScript has evolved to be executed on the server and almost on any device consisting of the JavaScript Engine: since 2012, all modern web browsers have implemented the ECMAScript 5.1 version of JavaScript. 

Every JS program (script) can be attached to any web page’s HTML and runs when the page loads. Since the release of Node.js, JavaScript has been a versatile language to build server-side software, and even big companies such as Facebook or Google use it to develop desktop-like web applications. 

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