Frequently Asked Questions for JavaScript Interview

Top 40 Frequently Asked Questions for JavaScript Interview

As JavaScript is so versatile in front-end and back-end development, mobile app development, has a sizable community and ecosystem, and is always evolving, there is a huge demand for JavaScript developers. Here we have crafted the top 40 most common questions for JavaScript interviews. Get started with our JavaScript course syllabus.

JavaScript Interview Questions for Freshers

Here are the basic JavaScript interview questions and answers for freshers.

1. What is JavaScript?

The main purpose of JavaScript, a high-level, interpreted programming language, is to create interactive web pages. It is one of the core technologies of the World Wide Web, along with HTML and CSS.

2. Describe how “==” and “===” vary in JavaScript.

== (Loose equality): It verifies equality following type coercion (For example, “5” == 5 will evaluate to true).

=== (strict equality): Value and type equality are checked using === (strict equality); for example, “5” === 5 will evaluate to false)

3. What are the different data types in JavaScript?

The various types of data types in JavaScript are

Primitive:

• Number
• String
• Boolean
• Null
• Undefined
• Symbol (ES6)
• BigInt (ES10)

Object:

• Arrays
• Objects
• Functions

4. What does JavaScript’s hoisting mean?

The JavaScript technique known as “hoisting” involves moving variable and function declarations to the top and fully lifting them. Although variable declarations are raised, their assignments stay in place.

5. How do var, let, and const vary from one another?

6. Describe the JavaScript closures.

A closure is a function that, even after the outer (enclosing) function has completed running, retains access to variables from its scope.

7. What are the various ways to define a function in JavaScript?

Option 1: 

Function Declaration:

JavaScript

function myFunction() { 

    // function body 

}

Option 2: 

Function Expression:

JavaScript

const myFunction = function() { 

    // function body 

}; 

Option 3: 

Arrow Function:

JavaScript

const myFunction = () => { 

    // function body 

}; 

8. What does JavaScript’s “this” keyword mean?

The object of which the function is a property is denoted by the keyword this. Depending on how the function is called, its value may change.

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9. What is the DOM?

HTML and XML documents can be programmed using the Document Object Model (DOM). It lets JavaScript work with and modify page elements by representing the page as a tree-like structure of nodes.

10. In JavaScript, what are event listeners?

Functions known as event listeners are those that run in reaction to a particular event, like a keystroke, mouseover, or click.

11. Describe how to use arrays with the “forEach” technique.

The forEach() method iterates through an array’s elements, calling a specified callback function for each one.

12. Which methods are available for adding elements to an array?

Push(): Appends items to the array’s end.

unshift(): Appends elements to the array’s start.

splice(): Adds elements at a specified index using the splice() function.

13. How do you remove elements from an array?

pop(): Gets rid of the final element and returns it.

shift(): Gets rid of the first element and gives it back.

splice(): Elements at a certain index are removed using the splice() function.

14. Describe how synchronous and asynchronous JavaScript vary from one another.

Synchronous: Line by line, the code is run, with each statement finishing before the next one begins.

Asynchronous: By using setTimeout, Promises, and async/await, for example, asynchronous programming enables code to run separately without preventing the main thread from being blocked.

15. What do JavaScript promises mean?

An asynchronous operation’s ultimate success (or failure) and the value that results are represented by a promise.

16. Describe how to use arrays with the “map” technique.

By applying a specified function to each element of the original array, the map() method generates a new array.

17. How do the “filter” and “reduce” approaches vary from one another?

filter(): It generates a new array with just the elements that pass a function-provided condition.

reduce(): Reduces an array to a single value by applying a function to an accumulator and each value (from left to right).

18. What is the “try…catch…finally” block used for?

• try: Contains the code that could result in an error.
• catch: Deals with the error if it happens.
• finally: code that will run consistently whether or not there was an error.

19. What is JSON?

Machines can easily parse and generate JSON (JavaScript Object Notation), which is a lightweight data-interchange format that is easy for humans to read and write. Data transmission between a web application and a server is one of its common uses.

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20. What distinguishes JavaScript’s “null” from “undefined”?

null: A value that is purposefully absent. The value is assigned.

Undefined: Denotes a declared variable without a value given to it.

JavaScript Interview Questions for Experienced

Here are the advanced JavaScript interview questions and answers for experienced candidates.

21. What function does JavaScript’s “window” object serve?

The window object is a representation of the browser window. Numerous browser-related properties and methods, including location, document, setTimeout, and others, are accessible through it.

Functions of “Window” Objects:

• Manages the browser: You may manage the browser environment by using the window object to start new windows, resize existing ones, and navigate through the history of the browser.
• Display alerts: The window object offers ways to show prompt boxes, confirm boxes, and alarms.
• Information retrieval: The window object offers ways to get details about the browser, like the size of the screen, the type of browser, and the pages visited.
• Alters the document: The window object offers ways to work with the document, including adding text to the page and scrolling through it.
• Schedule tasks: Methods for scheduling tasks, like setting up a function to run every few milliseconds, are available in the window object.  

22. Which JavaScript frameworks are popular?

Several well-known JavaScript frameworks are as follows:

• React: A user-friendly, adaptable, and powerful front-end framework. It is renowned for its virtual DOM and component-based architecture.
• Vue.js: An excellent framework for developing cross-platform applications and technologies. It’s a popular option for big businesses.
• Ember.js: 2011 saw the debut of the open-source framework Ember JS. It is regarded as an excellent option for developing online applications.
• Angular: One framework that works well for creating single-page applications (SPAs) is called Angular. Google is the developer of it.
• Node.js: A performance-oriented framework. Because it is event-driven, it can manage several requests concurrently.
• AngularJS: A well-liked framework that offers a thorough framework for front-end web development. It made it possible for developers to create reusable parts.

23. What is the “addEventListener” method used for?

An event handler is attached to a specific element using the addEventListener() function. It lets you define the kind of event (like “click,” “mouseover”) and the action that will be taken when the event takes place.

24. In JavaScript, how do you create a cookie?

By setting the document, you can make a cookie.the appropriate name, value, and expiration date for the cookie attribute.

Code: 

document.cookie = “username=Rinki; expires=Thu, 18 Dec 2023 12:00:00 UTC; path=/”;

username=Rinki: Sets the value to Rinki and the cookie’s name to username. 

expires=Thu, 18 Dec 2023 12:00:00 UTC: determines the cookie’s expiration date.

path=/: Determines the URL of the website where the cookie is active.

25. What are the “sessionStorage” and “localStorage” objects used for?

localStorage: Data that has no expiration date is stored locally.

sessionStorage: Data that is only accessible for the length of the current session until the browser window is closed.

26. What distinguishes “setAttribute” from “getAttribute”?

getAttribute(): Gets an attribute’s value from an element.

setAttribute(): Modifies an element’s attribute value.

27. Describe the idea of event capture and event bubbling.

By default, events “bubble up” the DOM tree from the target element to its parent elements, a phenomenon known as event bubbling. An alternate method is event capturing, in which events “capture” the path from the root element to the target element in the DOM tree.

Event capturing:

• The initial stage of the spread of an event
• It allows parent elements to handle events first and begins at the base of the DOM tree before moving down to the target element. 
• It is useful for catching events before they reach their intended target.
• By default, event capture is turned off.

Event bubbling:

• The second stage of the spread of an event
• It enables child elements to handle events before their parents by beginning at the target element and moving up to the DOM tree’s root.
• It is beneficial for methods such as event delegation
• Events “bubble” up via the parents from the inner element, which is why the process is called “bubbling.”  

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28. What is a callback function?

The definition of a callback function is a function that is run inside another function after being supplied as an argument. 

How they work: After passing the callback function to the API, the caller eventually runs the callback function. The caller is responsible for providing the proper parameters to the callback function.

When they’re utilized: Callbacks are frequently used to manage asynchronous events, such as file I/O or network requests. Additionally, they can be used to improve the readability and modularity of code.

Callbacks can be triggered either synchronously or asynchronously. Asynchronous callbacks are called after an asynchronous operation is finished, whereas synchronous callbacks are called just after the outer function is called.  

29. Describe how the “spread syntax” is used (…)

An iterable (such as an array or string) can be expanded into individual elements using the spread syntax. Arrays can be copied using

Copy arrays: const newArray = […oldArray];

Concatenate arrays: const combined = […array1, …array2];

Pass arguments to functions: myFunction(…args);

How it’s used

• Function calls: You can send zero or more arguments to a function using the spread syntax. For example, sum(…numbers).
• Array literals: You can add zero or more elements to an array using the spread syntax. For example, let dest = […src]
• Object literals: You can add zero or more key-value pairs to an object using the spread syntax. For example, let obj3 = {…obj1, …obj2}. 

30. What is the “async/await” syntax used for?

The async/await syntax offers a more sophisticated approach to using Promises for asynchronous activities.

• async: A function that has async before it returns a promise.
• await: When a promise is made, the function must wait for the promise to be fulfilled before proceeding.

Examples:

Async/await in Node.js can be used to handle time-consuming operations, such as awaiting an API response.

Async/await allows web applications to handle more requests with the same number of threads.  

31. What do JavaScript modules mean?

JavaScript code can be arranged and contained into reusable chunks using modules. They let you import and export variables, classes, and functions between files.

32. Why are the keywords “import” and “export” used?

• To import modules or certain members from other modules, use the import function.
• To enable functions, classes, or variables to be imported by other modules, use the export function.

33. Which design patterns are frequently used in JavaScript?

The following are a few common JavaScript design patterns:

Singleton: A creational pattern that restricts a class’s instances to one. If you wish to restrict access to a shared instance, this pattern can be helpful.

Observer: A pattern of behavior that alerts objects automatically when another object’s state changes. The loose connection is encouraged by this arrangement.

Prototype: A creational pattern that copies a prototype instance to produce objects. When you need to generate numerous instances of an object with shared state, this technique can be helpful.

Decorator: A structural pattern that establishes connections between items and offers adaptable methods for modifying their arrangement.  

Facade: A structural pattern that establishes connections between items and offers adaptable methods for modifying their arrangement.

Adapter: A structural pattern that establishes connections between items and offers adaptable methods for modifying their arrangement.

Bridge: A structural pattern that establishes connections between items and offers adaptable methods for modifying their arrangement.

Composite: A structural pattern that establishes connections between items and offers adaptable methods for modifying their arrangement.

34. What distinguishes an object in JavaScript from a class?

Class: An object-creation blueprint or template.

Object: An object is a specific instance of a class, complete with unique methods and properties.

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35. What are the various JavaScript object creation methods?

Object literal: const myObject = { name: “John”, age: 30 };

Constructor function: function Person(name, age) { this.name = name; this.age = age; }

Class: class Person { constructor(name, age) { this.name = name; this.age = age; } }

36. What does JavaScript’s “prototype” serve as?

You can add shared methods and attributes to every instance of a constructor function by using its prototype property.

The purpose of prototype:

• Inheritance: Objects can inherit methods and properties from other objects thanks to prototypes. Both a professor and a student, for instance, may inherit from a person object, but they may also have unique characteristics tailored to their respective roles.
• Extending functionality: An object can have new variables or methods added to it using prototypes. This allows all instances of the object to use the new variable or method.
• Memory efficiency: By consolidating shared methods into a single object, prototypes can help prevent memory waste.

The prototype of the constructor function that generated the object is assigned to the object’s prototype upon creation.

JavaScript verifies the object itself before calling a property or function on it. It verifies the object’s prototype if the property or method cannot be found, and so on.

A new object with a customized prototype can be made using the Object.create() method.

37. In JavaScript, what distinguishes a function from a method?

A method is a function that is connected to an object in JavaScript, whereas a function is a block of code that may be defined and invoked. The primary distinction is that whereas a function lacks access to an object’s properties and methods, a method does.  

Functions: 

• Functions are reusable code segments that carry out certain tasks.
• Functions are able to return output values after accepting input arguments.
• Functions provide reusability and modularity. 

Method: 

• Functions that are called on objects are called methods.
• The properties of the object can be accessed or changed by methods.
• Behavior is encapsulated within objects via methods.
• Building reliable, maintainable object-oriented systems requires methods.

Examples

• The function add(a, b) returns the total of two parameters.
• A string object is converted to uppercase by calling the toUpperCase() function. 

Developers can benefit from knowing the distinction between functions and methods. Make use of the right tool for the job. Write code that is clear and effective, and make it easier to read and organize.

38. What is the “bind” method used for?

Regardless of how the function is called, the bind() method generates a new function that, when called, has its keyword set to a certain value.

39. What are some common methods for debugging JavaScript?

Common methods for JavaScript debugging include:

Breakpoints: Stop code execution at a designated line to examine the application’s current state.

Console logging: Values for variables, props, and states are printed out at various code locations.

Console: Print messages, record faults, track performance, and examine objects in real time with the console object.

Debugger: To halt code execution while it is running, insert a debugger line.

Conditional breakpoints: Breakpoints that are conditional: In complex code, utilize breakpoints to halt execution at particular lines.

Event listener breakpoints: To halt code execution at particular event listener points, use breakpoints. 

40. What are some best practices for creating JavaScript code that is clear and easy to maintain?

• Use sensible names for your variables and functions.
• Write remarks that are succinct and clear.
• Observe uniform code style standards.
• Divide difficult issues into smaller, more manageable components.
• Make sure you thoroughly test your code.

Conclusion

JavaScript expertise can lead to a variety of fascinating job options with attractive salaries for prospective developers. People can put themselves in a successful position in this exciting and fulfilling industry by grasping fundamental ideas, picking up popular frameworks, and consistently improving their abilities. We hope these top 40 questions for the JavaScript interview will help you review your skills. Leverage our JavaScript training in Chennai to kickstart your development career.