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Top 50 JavaScript Interview Questions and Answers

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Whether you’re a fresher or an experienced developer, JavaScript is a critical component of your web development toolkit. So, it’s essential to get well-prepared for your JavaScript interviews. This post shares the top 50 JavaScript interview questions and answers, along with practical examples. Let’s dive in!

1. What is JavaScript?

JavaScript is a high-level, interpreted scripting language. It’s one of the core technologies of the World Wide Web, alongside HTML and CSS. JavaScript enables interactive web pages and is essential for web applications. Learn more about JavaScript.

2. Explain the difference between JavaScript and ECMAScript.

JavaScript is a scripting language, while ECMAScript is the standard that governs the implementation of JavaScript. JavaScript is based on ECMAScript and adds additional features not described in the ECMAScript standards.

3. What are JavaScript data types?

JavaScript supports several data types including: undefined, null, boolean, string, symbol, number, and object.

4. Can you explain the concept of ‘this’ in JavaScript?

The ‘this’ keyword in JavaScript refers to the object that the function is a method of. The value of ‘this’ is determined by how a function is called. It’s a tricky part of JavaScript, as it behaves differently from most other programming languages.

5. What is JavaScript hoisting?

In JavaScript, variable and function declarations are hoisted to the top of their containing scope. It’s a unique feature of JavaScript. However, only the declarations are hoisted, not the initializations.

6. What is a closure in JavaScript?

A closure in JavaScript is a function that remembers its outer variables and can access them. In other words, a closure gives you access to an outer function’s scope from an inner function.

7. What’s the difference between ‘null’ and ‘undefined’ in JavaScript?

‘Undefined’ means a variable has been declared but has not yet been assigned a value. On the other hand, ‘null’ is an assignment value that means no value or no object. It is an intentional absence of any value.

8. What are JavaScript Promises?

JavaScript Promise is an object representing the eventual completion or failure of an asynchronous operation. It serves as a link between the function that’s executing and the functions consuming its result.

9. Can you explain ‘Prototype Inheritance’ in JavaScript?

Prototype Inheritance is a form of object-oriented inheritance in JavaScript where methods and properties are added to the prototype properties of the constructor functions, rather than directly to the object.

10. What is the use of the ‘new’ keyword in JavaScript?

The ‘new’ keyword in JavaScript is used to create an instance of an object. When ‘new’ is used, it creates a new empty object, and then calls the function, assigning ‘this’ to the new object.

11. Explain event bubbling in JavaScript?

Event bubbling is a way of event propagation in the HTML DOM API, when an event occurs in an element inside another element, and both elements have registered a handle for that event.

12. What is the difference between ‘==’ and ‘===’ operators in JavaScript?

The ‘==’ operator checks for equality in value but not type, meaning it performs type coercion if values are not of the same type. In contrast, the ‘===’ operator checks for both value and type, so it doesn’t perform type coercion.

13. What are JavaScript Arrow Functions?

Arrow functions, introduced in ECMAScript 6, provide a concise way to write functions in JavaScript. Unlike regular functions, arrow functions don’t create their own ‘this’ value. They are especially useful for writing callback functions in a concise manner.

14. What does ‘use strict’ do in JavaScript?

The ‘use strict’ directive was introduced in ECMAScript 5. It changes the JavaScript parser’s behavior to a stricter interpretation, preventing certain actions from being taken and throwing more exceptions.

15. Can you explain ‘async/await’ in JavaScript?

‘async/await’ is a modern way to handle asynchronous operations in JavaScript. An ‘async’ function returns a promise, and the ‘await’ keyword is used to wait for the promise to be resolved or rejected.

Whether you're a fresher or an experienced developer, JavaScript is a critical component of your web development toolkit. So, it's essential to get well-prepared for your JavaScript interviews. This post shares the top 50 JavaScript interview questions and answers, along with practical examples. Let's dive in!

16. What is NaN in JavaScript?

‘NaN’ stands for Not a Number. It’s a special value that indicates a missing numerical result. For example, trying to parse a non-numeric string into a number would result in NaN.

17. Explain how JavaScript handles AJAX requests.

AJAX stands for Asynchronous JavaScript and XML. It’s a technique for accessing web servers from a web page. AJAX allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes. This means that it’s possible to update parts of a web page without reloading the whole page.

18. What is the difference between ‘let’, ‘const’, and ‘var’?

‘var’, ‘let’, and ‘const’ are all used for variable declaration in JavaScript. However, they have different scope rules. ‘var’ is function-scoped, ‘let’ and ‘const’ are block-scoped. Additionally, ‘const’ declares a read-only reference to a value. It does not mean the value is immutable, just that the variable identifier cannot be reassigned.

19. What are JavaScript’s ‘Truthy’ and ‘Falsy’ values?

‘Truthy’ and ‘Falsy’ are terms used in JavaScript to describe truthiness or falsiness of a value when evaluated in a Boolean context. ‘Falsy’ values are values that evaluate to false. These include 0, ”, null, undefined, NaN, and boolean false. Everything else in JavaScript is considered ‘Truthy’.

20. What is a JavaScript Callback Function?

A callback function is a function that is passed as an argument into another function, to be invoked/called back at a later time. Callbacks are often used in asynchronous code to ensure that certain code doesn’t execute until other code has finished execution.

21. How can you create an object in JavaScript?

There are several ways to create an object in JavaScript:
1. Using an object literal: e.g., let obj = {};
2. Using the Object constructor: e.g., let obj = new Object();
3. Using Object.create(): e.g., let obj = Object.create(null);

22. What is JSON and how does it relate to JavaScript?

JSON stands for JavaScript Object Notation. It’s a lightweight data-interchange format that’s easy to read and write for humans and easy to parse and generate for machines. Although inspired by JavaScript, it is language-independent and is used to transmit data between the server and the web application, serving as an alternative to XML.

23. What is the DOM in JavaScript?

The DOM (Document Object Model) is a programming interface for web documents. It represents the structure of a document and allows a way to manipulate the content and structure of a web page. In JavaScript, it is used to change the document structure, style, and content.

24. Explain JavaScript’s event loop.

The Event Loop is a queue of callback functions. When an async function executes, the callback function is pushed into the queue. The JavaScript engine doesn’t start processing the event loop until the code after an async function has executed.

25. Can you explain ‘IIFE’ in JavaScript?

IIFE stands for Immediately Invoked Function Expression. It’s a JavaScript function that runs as soon as it is defined. The syntax is: (function(){ /* code */ })(). It’s often used to create a new scope to avoid polluting the global scope.

26. What is an API in JavaScript?

An API, or Application Programming Interface, in JavaScript, is a set of functions and procedures that allow the creation of applications that access the features or data of an operating system, application, or other services.

27. What is the ‘spread operator’ in JavaScript?

The spread operator (…) in JavaScript is used to expand iterable elements. You can use it with arrays and objects to create a copy, merge, or expand elements.

28. What are template literals in JavaScript?

Template literals are string literals allowing embedded expressions. Introduced in ECMAScript 6, they are enclosed by the back-tick (` `) character instead of double or single quotes. They can contain placeholders, indicated by the dollar sign and curly braces (${expression}).

29. Can you explain the concept of destructuring in JavaScript?

Destructuring is an ES6 feature that allows you to extract multiple properties from an object or array in a single statement. It can greatly simplify your code, making it more readable and maintainable.

30. What is the Map object in JavaScript?

A Map is a collection of elements where each element is stored as a Key, Value pair. Map objects remember the original insertion order of the keys and can have objects or primitive values as keys or values.

31. Explain Set in JavaScript.

A Set is a special type collection – “set of values” (without keys), where each value may occur only once. It has methods for adding, removing, and checking elements.

32. What are JavaScript classes?

JavaScript classes, introduced in ECMAScript 2015, are primarily syntactical sugar over JavaScript’s existing prototype-based inheritance. A class definition sets enumerable flag to false for all methods in the “prototype”.

33. How can you handle errors in JavaScript?

Errors in JavaScript can be handled using try-catch-finally blocks. The try block contains the code segment which might throw an error, the catch block will execute when an error is thrown, and the finally block will execute regardless of an error being thrown or not.

34. What is the difference between ‘forEach’ and ‘map’ methods in JavaScript?

Both ‘forEach’ and ‘map’ are methods used to iterate over the elements of an array. The main difference between them is that ‘forEach’ doesn’t return a value, while ‘map’ creates a new array with the results of calling a provided function on every element in the calling array.

35. What is the ‘Global Object’ in JavaScript?

The Global Object in JavaScript refers to the window object in a browser environment and to the global object in a Node.js environment. All global variables belong to the global object.

36. What is a ‘Singleton’ in JavaScript?

A Singleton is an object which can only be instantiated once. A singleton pattern restricts the instantiation of a class to a single instance and provides a global point of access to it.

37. Explain the ‘slice’ and ‘splice’ methods in JavaScript.

‘Slice’ and ‘splice’ are array methods in JavaScript. ‘Slice’ is used to copy a portion of an array into a new array, without modifying the original array. On the other hand, ‘splice’ is used to add/remove items to/from an array, and returns the removed item(s).

38. What is ‘Web Storage’ in JavaScript?

Web Storage, also known as HTML5 Storage, Local Storage or DOM Storage, offers a way for web applications and pages to store data persistently in the browser’s memory. It’s more secure, and large amounts of data can be stored locally, without affecting website performance.

39. What is an ‘Observer’ in JavaScript?

An ‘Observer’ is part of the observer design pattern. It’s a design pattern in which an object (known as the subject) maintains a list of objects depending on it (observers), automatically notifying them of any changes in state.

40. What is the importance of JavaScript hoisting?

Hoisting is JavaScript’s default behavior of moving all declarations to the top of the current scope (to the top of the current script or the current function). It’s important because it allows us to invoke a function before it has been declared in the code.

41. How does ‘this’ keyword work in JavaScript?

The JavaScript ‘this’ keyword refers to the object it belongs to. It has different values depending on where it is used. In a method, ‘this’ refers to the owner object. Alone, ‘this’ refers to the global object. In a function, ‘this’ refers to the global object. In an event, ‘this’ refers to the element that received the event.

42. What is ‘type coercion’ in JavaScript?

Type coercion is the automatic or implicit conversion of values from one data type to another (such as string to a number). JavaScript is a dynamically typed language. That means you don’t have to specify what type of information will be stored in a variable.

43. Can you explain the ‘rest parameter’ in JavaScript?

The rest parameter syntax allows us to represent an indefinite number of arguments as an array. With the help of a rest parameter, a function can be called with any number of arguments, no matter how it was defined.

44. What is a generator in JavaScript?

Generators are functions which can be exited and later re-entered. Their context (variable bindings) will be saved across re-entrances. Generators in JavaScript — especially when combined with Promises — are a powerful tool for asynchronous programming as they mitigate the problems with callbacks.

45. Can you explain the difference between window, screen and document in JavaScript?

The window is the first thing that gets loaded into the browser. This window object has the majority of the properties like length, innerWidth, innerHeight, name, if it has been closed, its parents, and more.
The screen object has information about the user’s screen resolution.
And finally, the document object is your html, aspx, php, or other document that will be loaded into the browser.

46. How do JavaScript primitive/object types passed in functions?

Primitive types like numbers and booleans are passed by value, while objects (arrays, functions, and objects) are passed by reference. If you change a property of the passed object, the change will be visible outside the function.

47. What is a JavaScript framework, and why would you use one?

A JavaScript framework is an application framework written in JavaScript where the developers can manipulate the functions and use them for their convenience. JavaScript frameworks are a type of tool that makes working with JavaScript easier and smoother.

48. What are Promises in JavaScript?

A Promise in JavaScript is an object representing the eventual completion or failure of an asynchronous operation. Essentially, it is a returned object to which you attach callbacks, instead of passing callbacks into a function.

49. Explain how prototypal inheritance works in JavaScript.

Prototypal inheritance is a feature in JavaScript in which an object can inherit properties from another object. JavaScript uses internal [[Prototype]] property for prototypal inheritance. When trying to access a property of an object, the property will not only be sought on the object but on the prototype of the object, the prototype of the prototype, and so on until either a property with a matching name is found or the end of the prototype chain is reached.

50. Can you explain what ‘closures’ are in JavaScript?

A closure in JavaScript is a function that has access to its own scope, the scope of the outer function, and the global scope. The closure has access to variables in three separate scopes, to the parameters of the outer function(s), and to the global variables. It is a powerful feature in JavaScript programming that makes it possible to write more efficient and flexible code.

Understanding these top 50 JavaScript interview questions and their answers will give you a significant edge in your interviews. Stay curious, keep learning, and always challenge yourself to improve your JavaScript knowledge and skills.



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