35+ ESSENTIAL JavaScript Interview Question & Answer

Last updated on 03rd Jul 2020, Blog, Interview Questions

JavaScript, a pivotal component of web development, is a versatile and widely-used programming language renowned for its client-side capabilities. It operates within web browsers, rendering web pages interactive and dynamic. As a high-level and interpreted language, it obviates the need for pre-compilation, offering a straightforward development experience. What makes JavaScript particularly powerful is its versatility, accommodating various programming paradigms, including object-oriented, functional, and imperative styles. It lies at the heart of web development, working in tandem with HTML and CSS to craft interactive forms, animations, and real-time updates. Governed by the ECMAScript standard, JavaScript continually evolves with different versions and updates, shaping its language features and capabilities. However, it does require vigilant attention to cross-browser compatibility, as various browsers may interpret and execute JavaScript code differently, presenting a challenge for developers. Nevertheless, JavaScript remains an indispensable tool for creating engaging and responsive web experiences.

1. What is JavaScript?

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  JavaScript is a versatile, high-level, and interpreted programming language for creating dynamic web content and applications.

2. Explain the difference between JavaScript and Java.

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JavaScript is a lightweight, interpreted scripting language primarily used for web development to add interactivity and behavior to websites. At the same time, Java is a compiled, high-level programming language used for various applications, including web and desktop software. JavaScript runs in web browsers, enabling dynamic web page functionality, while Java is platform-independent, typically used for server-side applications, Android app development, and other non-web software. Despite their similar names, JavaScript and Java have different syntax, purposes, and use cases. The development of front-end web pages requires JavaScript, and Java is a versatile language for diverse software development scenarios.

3. What are the data types in JavaScript?

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JavaScript has six primitive data types: undefined, null, boolean, number, string, and symbol. Additionally, it has a complex data type, object.

4. What is the JavaScript syntax for declaring variables?

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You can declare a variable using var, let, or const followed by the variable name.

5. What is the difference between let, const, and var for variable declaration?

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let and const have block scope, while var has function scope. const is used for variables that should not be reassigned, whereas let allows reassignment.

6. What is a JavaScript function?

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A JavaScript function is a reusable block of code defined with the function keyword. It can take parameters as inputs, executes a specific set of instructions enclosed in curly braces {}, and may return a value using the return statement. Functions are essential for organizing and encapsulating code logic, making it modular and maintainable in JavaScript applications.

7. How is a function defined in JavaScript?

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A function can be defined by using the function keyword together with the function name and parameters.

8. Explain the concept of closures in JavaScript.

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Closures are a fundamental and powerful concept in JavaScript. They occur when a function “closes over” or retains access to variables from its outer (enclosing) scope, even after the outer function’s execution is complete. This means that the inner function can still access and manipulate the variables from the outer function, even when the outer function’s execution context is no longer active.

9. What is callback hell in JavaScript?

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Callback hell, also called “Pyramid of Doom”, refers to deeply nested callback functions, making code harder to read and maintain.

10. What is an object in JavaScript?

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An object is a group of key-value pairs, where a key is a string (or symbol) and a value is any sort of data.

11. What is an array in JavaScript?

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A collection of elements, which can be of any data type, are stored in an array, which is a form of data structure.

12. How do you add elements to an array in JavaScript?

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By manually assigning a value to an index or by using functions like push() and unshift(), you can add elements to an array.

13. What does JavaScript’s Document Object Model (DOM) stand for?

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A programming interface for web documents is called the DOM. It enables interaction between JavaScript and HTML and CSS by representing the page’s structure and content.

14. How do you select elements in the DOM using JavaScript?

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You can select elements by their tag name, class, ID, or other attributes using methods like getElementById(), querySelector(), and getElementsByClassName().

15. Explain the global scope and local scope in JavaScript.

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Global scope refers to variables accessible throughout the program, while local content pertains to variables only accessible within a specific function or block.

16. What is the “this” keyword in JavaScript?

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This refers to the current object or context, depending on how and where it is used. It can change its value based on the execution context.

17.  What does JavaScript’s type coercion mean?

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The automatic conversion of values across data types during operations is known as type coercion.

18. What distinguishes JavaScript’s null and undefined values?

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Undefined denotes a variable or object attribute that has not been given a value, while null denotes the purposeful absence of any object value.

19. Explain async/await in JavaScript.

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A more elegant syntax for writing asynchronous code is async/await. While the await keyword is used to halt execution until a Promise is resolved, the async keyword is used to declare an asynchronous function.

20. What is scope in JavaScript?

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Scope defines the accessibility of variables. JavaScript has function scope and block scope (with let and const).

21. In JavaScript, what does hoisting mean?

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Variable and function declarations are hoisted in JavaScript, which occurs during compilation and moves them to the top of their containing scope.

22. Explain the global scope and local scope in JavaScript.

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Global scope refers to variables accessible throughout the entire program, whereas variables having a limited access within a single function or block are under the category of local scope.

23. What is the “this” keyword in JavaScript?

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this refers to the current object or context, depending on how and where it is used. It can change its value based on the execution context.

24. What does JavaScript’s type coercion mean?

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Type coercion is the automatic conversion of values from one data type to another during operations.

25. What distinguishes JavaScript’s null and undefined ?

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Null denotes an object’s purposeful absence of any value,while undefined indicates a variable or object property that has not been assigned a value.

26. What is NaN, and how do you check if a value is NaN?

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NaN (Not-a-Number) is a special value indicating an unrepresentable mathematical result. You can check for NaN using the isNaN() function or the Number.isNaN() method.

27. What are the different types of operators in JavaScript?

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JavaScript has arithmetic, comparison, logical, assignment, and other types of operators.

28. What is the difference between == and === in JavaScript?

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== checks for equality with type coercion, while === checks for strict equality (value and type).

29. What is the ternary (conditional) operator in JavaScript?

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The ternary operator (condition ? expr1 : expr2) is a shorthand way to write an if…else statement.

30. How do you use then() and catch() with Promises?

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then() is used to handle successful Promise resolutions, while catch() is used to handle errors or rejections.

31. What is Promise chaining in JavaScript?

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Promise chaining is a technique where multiple then() methods are used sequentially to process values returned by resolved Promises.

32. What is a JavaScript module, and why is it useful?

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A module is a unit of independently exportable code. and imported to separate and organize code in larger applications.

33. How do you export and import modules in JavaScript?

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You can use the export and import statements to export and import functionality between modules.

34. What are CommonJS and ES6 modules, and how do they differ?

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CommonJS is a module system used in Node.js, while ES6 modules are a standardized module system for both browsers and Node.js.

35. What is object-oriented programming (OOP) in JavaScript?

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OOP is a style of programming that makes use of objects and classes to model and structure code.

36. Describe the JavaScript idea of prototypal inheritance.

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Prototypal inheritance allows objects to inherit properties and methods from other objects through their prototype chain.

37. What is a constructor function in JavaScript?

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A constructor function is used to create and initialize objects with shared properties and methods.

38.  What is an asynchronous function in JavaScript?

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An asynchronous function is one that does not block the main thread, allowing other tasks to execute concurrently.

39.  What is the Event Loop in JavaScript, and how does it work?

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The Event Loop is a core concept in JavaScript that manages the execution of asynchronous code, ensuring non-blocking behavior.

40. What are Web Workers in JavaScript, and how do they enable parallelism?

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JavaScript code can be run in the background using Web Workers, which allows for effective parallelism and multi-threading in web applications.

41. What is the DOM (Document Object Model) in JavaScript?

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The DOM is a programming interface for web documents. It represents the structure of an HTML document as a tree of objects.

42. How do you create, modify, and delete DOM elements using JavaScript?

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You can create new elements with createElement(), modify attributes and content with setAttribute() and textContent, and remove elements with removeChild().

43. Explain event handling in JavaScript, and how do you attach event listeners?

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Event handling involves responding to user actions like clicks or keyboard input. You can attach event listeners to elements using methods like addEventListener().

44. What is AJAX in JavaScript, and what does it stand for?

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AJAX (Asynchronous JavaScript and XML) is a technique for making asynchronous HTTP requests to a server and updating a web page without a full page reload.

45. What is the Fetch API in JavaScript, and how is it used for making HTTP requests?

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A more recent replacement for XMLHttpRequest is the Fetch API for making HTTP requests in JavaScript. It provides a more powerful and flexible way to work with data.

46. Explain the same-origin policy and how it impacts AJAX requests.

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The same-origin policy restricts web pages from making requests to a different domain. To access resources from different origins, you may use techniques like Cross-Origin Resource Sharing (CORS).

47. What is a JavaScript error object, and how can you catch and handle errors?

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Errors in JavaScript are represented by error objects. You can catch and handle errors using try…catch blocks.

48. What is the difference between throw and throw new Error() in JavaScript?

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throw is used to throw any expression, while throw new Error() is commonly used to throw error objects with a descriptive message.

49. What is a Promise in JavaScript?

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A Promise is a built-in object used for handling asynchronous operations. It represents a value that may not be available yet.

50.How do you create a Promise in JavaScript?  

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You can create a Promise by instantiating it with the new Promise() constructor and passing a function with resolve and reject callbacks.

51. What is the purpose of the then() and catch() methods with Promises?  

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The then() method is used to handle successful Promise resolutions, while catch() is used to handle errors or rejections.

52. What is a JavaScript module, and why is it useful?  

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An independent section of code is known as a module that can be exported and imported to separate and organize code in larger applications.

53. How do you export and import modules in JavaScript?  

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You can use the export and import statements to export and import functionality between modules.

54. What are CommonJS and ES6 modules, and how do they differ?

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CommonJS is a module system used in Node.js, while ES6 modules are a standardized module system for both browsers and Node.js.

55. What is a constructor function in JavaScript?

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A constructor function is used to create and initialize objects with shared properties and methods.

56. Describe the JavaScript idea of prototypal inheritance.  

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Prototypal inheritance allows objects to inherit properties and methods from other objects through their prototype chain.

57.  What is an asynchronous function in JavaScript?  

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An asynchronous function is one that does not block the main thread, allowing other tasks to execute concurrently.

58.  What is the Event Loop in JavaScript, and how does it work?  

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The Event Loop is a core concept in JavaScript that manages the execution of asynchronous code, ensuring non-blocking behavior.

59. What are Web Workers in JavaScript, and how do they enable parallelism?  

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To run JavaScript code in the background, use Web employees.Using Web Workers, JavaScript code can be executed in the background, enabling parallelism and efficient multi-threading in web applications.

60. What is the DOM (Document Object Model) in JavaScript?

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A programming interface for web documents is called the DOM. It displays the HTML document’s structure as a tree of objects.

61. How do you create, modify, and delete DOM elements using JavaScript?  

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You can create new elements with createElement(), modify attributes and content with setAttribute() and textContent, and remove elements with removeChild().

62.Explain event handling in JavaScript, and how do you attach event listeners?

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Event handling involves responding to user actions like clicks or keyboard input. You can attach event listeners to elements using methods like addEventListener().

63. What is AJAX in JavaScript, and what does it stand for?

Ans: 

AJAX (Asynchronous JavaScript and XML) is a technique for making asynchronous HTTP requests to a server and updating a web page without a full page reload.

64. What is the Fetch API in JavaScript, and how is it used for making HTTP requests?

Ans: 

The Fetch API is a modern alternative to XMLHttpRequest for making HTTP requests in JavaScript. It provides a more powerful and flexible way to work with data.

65.Explain the same-origin policy and how it impacts AJAX requests. 

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The same-origin policy restricts web pages from making requests to a different domain. To access resources from different origins, you may use techniques like Cross-Origin Resource Sharing (CORS).

66. How do you create a Promise in JavaScript?

Ans: 

You can create a Promise by instantiating it with the new Promise() constructor and passing a function with resolve and reject callbacks.

67. What is the purpose of the then() and catch() methods with Promises?  

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he then() method is used to handle successful Promise resolutions, while catch() is used to handle errors or rejections.

68.  What is a JavaScript module, and why is it useful?  

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A module is a unit of independently functioning code that can be exported and imported to separate and organize code in larger applications.

69.  How do you export and import modules in JavaScript?

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You can use the export and import statements to export and import functionality between modules.

70. What are CommonJS and ES6 modules, and how do they differ?

Ans: 

CommonJS is a module system used in Node.js, while ES6 modules are a standardized module system for both browsers and Node.js.

71. What is a constructor function in JavaScrip

Ans: 

A constructor function is used to create and initialize objects with shared properties and methods.

72. Explain the concept of prototypal inheritance in JavaScript.  

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Prototypal inheritance allows objects to inherit properties and methods from other objects through their prototype chain.

73.  What is mean by name function?

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The named function in JavaScript defines the name as well as value to a function. It is defined by using the keyword 

“function”

74. What is argument objects in javascript?

The arguments object in JavaScript is a special object available in functions, allowing access to all arguments passed to the function, regardless of the defined parameters. It behaves like an array-like object, making it useful for handling variable-length argument lists. However, it lacks array methods, so converting it to an array may be necessary for certain operations.

75. What is the Event Loop in JavaScript, and how does it work?

The Event Loop is a core concept in JavaScript that manages the execution of asynchronous code, ensuring non-blocking behavior.