Are you looking for JavaScript interview questions? You have arrived at the right place. JavaScript is an open-source scripting language designed for creating applications. It is lightweight, interpreted, and works faster than many other programming languages. It is the third biggest web technology to develop online and mobile applications, games, web servers, etc., that can be used in browsers.
In this article, we will mention some of the most popular and important JavaScript interview questions for beginners, intermediate, and advanced levels. So, let’s get started!
JavaScript Interview Questions For Beginners
Here are some JavaScript interview questions for beginners that you need to know:
Q: What is JavaScript?
Ans: JavaScript is a popular open-source programming and scripting language for enhancing web pages and creating an excellent user experience. It differs from Java language and is an object-based, cross-platform, translated, and lightweight language widely used for client-side scripting.
Q: What are the benefits of JavaScript?
Ans: Like other programming languages, JavaScript also has its advantages, which are as follows:
- Speed: It is an interpreted and client-side scripting language, which reduces the time needed by other programming languages, such as Java, for compilation.
- Simple: JavaScript is an easy-to-understand and learns, where the structure is relatively simple for both users and developers. It is relatively easy to execute, saving a lot of money to build creative and dynamic web content.
- Versatile: It is a versatile language capable of both front-end and back-end development. Back-end development uses NodeJS, while several other libraries help in front-end development, like AngularJS, ReactJS, etc.
- Less Overhead: JavaScript reduces the code length, which helps in enhancing the performance of websites and web applications. The codes comprise less overhead because of multiple built-in functions for DOM, loops, access, etc.
Q: What is a Function in JavaScript?
Ans: Functions in JavaScript are just like a process, a group of statements that performs a task or calculates a value. It is a set of reusable codes which can be called anywhere in the program, eliminating the need to repeat the code. In simpler terms, it enables the programmer to divide a big program into several small and convenient functions.
Example of Function Declaration
This is an example of Function Declaration to give you a brief understanding:
const abc = `Contract jobs ${domain}`;
console.log(abc);
}
Example of Function Expression
This is an example of Function Expression to give you a brief understanding:
const abc = `Contract jobs ${domain}`;
console.log(abc);
}
Q: List some essential features of JavaScript.
Ans: JavaScript is a popular language with multiple features when developing a web application. Some of them are as follows:
- Lightweight Scripting Language: It is a lightweight scripting language, as it is made for managing data on the browser. It is not a general-purpose language, having only considered a set of libraries. Besides, it is also developed for client-side execution for web applications; thus, it is lightweight.
- Stable Control: JavaScript offers stable control to the browser instead of being completely confided on the web servers. It offers multiple browsers with additional functionalities that help reduce server load and network traffic.
- Client-side Calculations: JavaScript is a client-side scripting language, it can perform simple calculations on the browser. The browser does not need to ask for permission for every task. It is highly helpful when a user needs to perform these calculations multiple times. In such cases, connecting to the server will take much more time than executing the actual calculations.
- Generating HTML Content: JavaScript has useful features for producing HTML content for the website. It allows the developers to add texts, tables, links, images, etc., after an event comes up.
Q: What is the difference between Java and JavaScript?
Ans: Here is the significant difference between Java and JavaScript:
| Java | JavaScript |
|---|---|
| Java is a strongly typed programming language. | It is a scripting language. |
| It is an object-oriented programming language. | It is an object-based language. |
| It is a standalone language. | It is not a standalone language because it needs to combine into an HTML program for implementation. |
| A strongly types language, making users choose the data type of a variable before declaring using it. | A loosely typed language makes users not bother about the variable’s data type before and after the declaration. |
| It should be merged before the execution. | It needs to be incorporated into the HTML program for implementation. |
| A difficult language to learn | An easy language to learn |
| Not required to run Java programs | Required to run JavaScript programs. |
| By using multi-threading, developers can perform difficult tasks | Users are not able to perform complicated tasks |
| It was developed by Sun Microsystems. | It was developed by Netscape. |
Q: What is BOM in JavaScript?
Ans: BOM, also known as the Browser Object Model, is one of the essential elements of JavaScript on the web. It offers all the objects and functionality and lets the developers interact with the browser.
Q: What is DOM in JavaScript?
Ans: DOM stands for Document Object Model, which is the data representation of the objects that includes the structure and content of a document on the web. In simple terms, it is a programming interface for web documents, representing the page so that programs can change the style, content, and document structure.
Q: What is the real name of JavaScript?
Ans: JavaScript was developed by Brendan Eich of Netscape Communications Corporation. It was originally named Mocha, then LiveScript, and again renamed to JavaScript.
Q: What is the use of a history object in JavaScript?
Ans: The JavaScript History is a built-in Interface used to access the browser history. It contains the list of links visited by the user in a specific browser window, frame, tab, etc. It is an essential property of the JavaScript window object, which can be accessed via the window.history property.
Q: What is an anonymous function in JavaScript?
Ans: It is a function without having any name. These functions are used differently during the runtime, using the function operator rather than the function declaration. It is relatively more flexible than function declaration and can be easily used in the place of an expression.
Javascript Interview Questions For Intermediate
Here are some JavaScript interview questions for Intermediate-level that you need to know:
Q1: What is the need for debugging in JavaScript?
Ans: Debugging is one of the essential and must-have skills for developers. It is the process of identifying the bugs and resolving them to enhance the efficient running of programs. JavaScript has multiple debugging strategies and tools to improve the performance of the code.
Q2: What is the role of a strict mode in JavaScript?
Ans: Strict Mode is a new feature that enables developers to place a function or program in a strict operating context. It prevents some particular actions from being taken, making JavaScript a safer option.
Q3: What is the use of Math Object in JavaScript?
Ans: Math is a built-in object with properties and methods for mathematical constants and functions, where its properties and methods are static. Unlike other objects, it has no constructor.
Q4: What is the meaning of hoisting in JavaScript?
Ans: Hoisting in JavaScript is a concept which allows you to remove values of variables and functions even before providing value without getting errors. It enables functions to be safely used in code before their declaration.
Q5: What are the types of scopes in JavaScript?
Ans: There are four types of scopes in JavaScript, which are as follows:
- Block Scope: Initially, JavaScript has Global and Function scope. let, and const are the two important keywords developed by ES6, which provide Block Scope in JavaScript. To access the variables of this specific block, you need to create an object for it.
- Local Scope: Local scope comprises things that are declared inside the code blocks and only accessible where it is declared.
- Function Scope: JavaScript also has a function scope, where each function creates a new scope. Variables explained inside a function cannot be accessed from outside the function, and variables declared with let, var, and const are just like when declared inside a function.
- Global Scope: Variables declared Globally, outside any function, can be accessed from anywhere in a program. These include Global Scope and Global variables, similar to function scope variables declared with var, let, and const, similar to when declared outside the block.
Q6: What is a callback in JavaScript?
Ans: Callback in JavaScript is a function that is passed to another function as an argument, which is then gathered inside the outset function to execute some routine and action. It is needed because multiple actions in JavaScript are asynchronous, which means they do stop the function from running till the time they are completed. Rather, it will run in the background while the rest of the code is still running.
Q7: What is the difference between Function declaration and Function expression?
Ans: Here is the significant difference between Function Declaration and Function Expression:
| Function Declaration | Function Expression |
|---|---|
| Also known as a function statement, declares a function with a function keyword. However, the function declaration must have a function name. | Just like a function declaration without having any function name. |
| Does not require a variable assignment because they are standalone constructs and cannot be put inside a functional block. | Can be stored in a variable assignment. |
| Executed before any other code. | Load and execute only when the program interpreter gets the code line. |
| Can be accessed before and after the function explanation. | Can be accessed only after the function definition. |
Q8: How can you add JavaScript code to HTML pages?
Ans: You can add JavaScript code to HTML pages in three ways, which are as follows:
- Embedding Code: For adding JavaScript code into HTML pages, you can use the <script>….</script> HTML tag that covers around JavaScript code inside the HTML program. You can also place JavaScript code in the body or head section, as it entirely relies on the design of the web page that developers use.
- Inline Code: This process is used to call a function in the HTML event attributes. There are multiple events or cases where developers have to add JavaScript code.
- External File: Developers can also make a separate file to keep the code of JavaScript with the (.js) extension and later add it to the HTML document. You can do it using the src attribute of the <script> tag. It helps maintain the web pages and also saves time from writing the same code again.
Q9: How can you delete a cookie in JavaScript?
Ans: You can delete a cookie in JavaScript in multiple ways, such as by using expire attribute, web browser, and max-age attribute.
Q10: What are the closures in JavaScript?
Ans: Closures are an essential JavaScript concept that every developer needs to know. It is a property of JavaScript that refers to independent variables, also known as free variables. It is a feature in which the inner function has access to the outer function variable. The three scope chains of closure are as follows:
- Access to the outer variables
- Access to its own scope
- Access to the global variables
JavaScript Interview Questions For Advanced Level
Here are some JavaScript interview questions for Advanced-level that you need to know:
Q: Name three JavaScript frameworks and their uses.
Ans: There are various JavaScript frameworks, and here are the most popular ones:
- Angular: It is a powerful JavaScript framework which works across multiple operating systems. It works by using HTML vocabulary on different web pages. Angular is a cross-platform framework, easy to comprehend and has fewer errors via its concise and data rendering technique. Apart from this, it can work on multiple operating systems.
- Vue JS: It is another JavaScript framework usually used in building cross-platform applications and websites. Other than that, it is also used to build Single Page Applications. It is flexible, small in size, easy to understand, and provides two-way communication.
- React JS: It is an open-source and one of the best JavaScript frameworks to create responsive and active UI (user interfaces). It is also SEO-friendly and highly recommended by developers who create e-commerce websites. Its key features include one-way data-binding only, Virtual Document Model (DOM), ability to test and debug quickly.
Q: What are object prototypes?
Ans: Prototypes are objects where JavaScript inherit features from one another. Simply, it is used to add new properties and methods to already existing objects and object constructors.
Q: What is memoisation?
Ans: Memoisation is one of the best techniques to make applications work faster and more effectively. It caches the results and retrieves the same information from the cache when it is needed next time for cashing instead of adding it again.
Q: What is the difference between the client-side and the server-side scripting?
Ans: Here are the significant differences between client-side and server-side scripting:
| Client-side Scripting | Server-side Scripting |
|---|---|
| The main purpose is to give the requested output to the end-user | The main purpose is to manipulate and offer access to the requested database |
| It is used at the front end, and the source code is visible to the user | It is employed at the backend, where the source code is not shown to the users as its output of server-side is an HTML |
| Does not need interaction with the server | Requires interaction with the server |
| Relatively less secure because of code accessibility given to the client | Highly secure while working on the web application as the code is not visible |
| Executed on the remote computer system | Executed on the web server |
| Highly depends on the user’s browser version | Does not depend on the client |
| Does not provide any access to the files on web | Provides complete access to the file that is stored in the web database server |
| Customise web pages and offer unique websites | Reduces server load |
Q: What is a WeakMap?
Ans: A WeakMap is a structure of key/value pairs that only allows objects as keys and removes them altogether with the related value once it cannot be accessed. It permits associating data to objects in a way that does not prevent the key objects from being collected, even if the values show the key.
Q: What is Event Bubbling in JavaScript?
Ans: Event bubbling is a strategy of event circulation in the HTML DOM API when an event is in an element that is inside another element, and both of those elements have registered a handle to that event. In simple words, it is an element that triggers the event and then bubbles up other elements in the library.
Q: State the difference between declaring variables using var, let, and cont.
Ans: Here are the major differences between var, let, and const:
| var | let | const |
|---|---|---|
| The scope of a varvariable in functional scope | The scope of a let variable is block scope | The scope of a const variable block scope |
| Can be updated and re-declared into the scope | Can be updated, but it cannot be re-declared into the scope | Cannot be updated or re-declared into the scope |
| Can be declared without initialisation | Can be declared without initialisation | Cannot be declared without initialisation |
| Can be accessed without initialisation because its default value is not defined | Cannot be accessed without initialisation, else it will give ‘referenceError’ | Cannot be accessed and declared without initialisation |
| Hoisting is done by initialising as default value | Hoisting is done, but not initialised | Hoisting is done but not initialised |
Q: What are undeclared and undefined variables?
Ans:
| Undefined Variable | Undeclared Variable |
|---|---|
| An undefined variable is a variable which has been declared but does not have a value. | An undeclared variable is a variable that does not even exist in the program. |
Q: What is Event Capturing in JavaScript?
Ans: Event capturing is one of the two ways of event distribution in the HTML DOM. In this process, the outermost element triggers the target element, which is the opposite of event bubbling.
Q: What is Strict mode in JavaScript?
Ans: Strict mode in JavaScript is a newly added feature which permits specific JS code to behave dynamically in a semantic order to secure it.
Conclusion
Now you know all the essential JavaScript interview questions, you can start preparing for them. Practice makes a man perfect, so sit in front of the mirror and ask yourself questions as if you are giving your interview. If you want to learn more about JavaScript, you can get yourself enrol in JavaScript training.
We hope you ace your interview and get the job of your dreams. All the best!
