shashankaz / javascript-questions-series

JavaScript Questions

Here are the 50 JavaScript interview questions for 2024

1. What is JavaScript?
2. What are the data types in JavaScript?
3. What is the difference between null and undefined?
4. Explain the concept of hoisting in JavaScript.
5. What is a closure in JavaScript?
6. What is the difference between “==” and “===” operators in JavaScript?
7. Explain the concept of prototypal inheritance in JavaScript.
8. What are the different ways to define a function in JavaScript?
9. How does event delegation work in JavaScript?
10. What is the purpose of the “this” keyword in JavaScript?
11. What are the different ways to create objects in JavaScript?
12. Explain the concept of callback functions in JavaScript.
13. What is event bubbling and event capturing in JavaScript?
14. What is the purpose of the “bind” method in JavaScript?
15. Explain the concept of AJAX in JavaScript.
16. What is the “typeof” operator used for?
17. How does JavaScript handle errors and exceptions?
18. Explain the concept of event-driven programming in JavaScript.
19. What is the purpose of the “async” and “await” keywords in JavaScript?
20. What is the difference between a deep copy and a shallow copy in JavaScript?
21. How does JavaScript handle memory management?
22. Explain the concept of event loop in JavaScript.
23. What is the purpose of the “map” method in JavaScript?
24. What is a promise in JavaScript?
25. How do you handle errors in promises?
26. Explain the concept of currying in JavaScript.
27. What is the purpose of the “reduce” method in JavaScript?
28. What is the difference between “null” and “undefined” in JavaScript?
29. What are the different types of loops in JavaScript?
30. What is the difference between “let,” “const,” and “var” in JavaScript?
31. Explain the concept of event propagation in JavaScript.
32. What are the different ways to manipulate the DOM in JavaScript?
33. What is the purpose of the “localStorage” and “sessionStorage” objects?
34. How do you handle asynchronous operations in JavaScript?
35. What is the purpose of the “forEach” method in JavaScript?
36. What are the differences between “let” and “var” in JavaScript?
37. Explain the concept of memoization in JavaScript.
38. What is the purpose of the “splice” method in JavaScript arrays?
39. What is a generator function in JavaScript?
40. How does JavaScript handle variable scoping?
41. What is the purpose of the “split” method in JavaScript?
42. What is the difference between a deep clone and a shallow clone of an object?
43. Explain the concept of the event delegation pattern.
44. What are the differences between JavaScript’s “null” and “undefined”?
45. What is the purpose of the “arguments” object in JavaScript?
46. What are the different ways to define methods in JavaScript objects?
47. Explain the concept of memoization and its benefits.
48. What is the difference between “slice” and “splice” in JavaScript arrays?
49. What is the purpose of the “apply” and “call” methods in JavaScript?
50. Explain the concept of the event loop in JavaScript and how it handles asynchronous operations.

Solutions

1. JavaScript: JavaScript is a high-level, interpreted programming language primarily used for front-end web development. It enables the creation of dynamic, interactive web pages by providing a scripting interface to the browser.

2. Data Types in JavaScript: JavaScript has several data types, including:

   • Primitive types: number, string, boolean, null, undefined, and symbol.
   • Object types: object, which includes arrays, functions, and objects.

3. Null vs. Undefined:

   • null is a deliberate assignment indicating the absence of a value.
   • undefined is a variable that has been declared but not assigned a value, or an object property that does not exist.

4. Hoisting in JavaScript: Hoisting is a behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase, allowing you to use them before they are declared.

5. Closure in JavaScript: A closure is a function that has access to variables from its outer (enclosing) scope even after the outer function has finished execution. It "closes over" the variables it needs.

6. "==" vs. "===" Operators:

   • == performs type coercion, attempting to make the operands the same type before making the comparison.
   • === checks for both value and type equality without coercion.

7. Prototypal Inheritance: JavaScript uses prototypal inheritance, where objects can inherit properties and methods from other objects through their prototype chains. Each object has a prototype object, and if a property is not found on the object itself, it looks up the chain.

8. Ways to Define a Function: Functions can be defined using function declarations, function expressions, arrow functions, and the Function constructor.

9. Event Delegation in JavaScript: Event delegation involves assigning a single event listener to a common ancestor, which then handles events on behalf of its descendants. This is particularly useful for handling events on dynamically created or large numbers of elements.

10. Purpose of "this" Keyword: In JavaScript, the this keyword refers to the object on which a method is being invoked or the object within which a function is being executed.

11. Ways to Create Objects: Objects can be created using object literals, constructor functions, the Object.create() method, and ES6's class syntax.

12. Callback Functions: Callback functions are functions passed as arguments to another function, to be executed later when a specific event occurs or a certain condition is met.

13. Event Bubbling and Capturing: Event bubbling is the process where an event starts from the target element and bubbles up the DOM hierarchy. Event capturing is the opposite, where the event is captured from the root and goes down to the target element.

14. Purpose of "bind" Method: The bind method in JavaScript is used to create a new function with a specified this value and initial arguments, ensuring that, when the new function is called, it has a specific context.

15. AJAX in JavaScript: AJAX (Asynchronous JavaScript and XML) is a technique for making asynchronous requests to a server from a web page, allowing data to be retrieved or sent without refreshing the entire page.

16. "typeof" Operator: The typeof operator in JavaScript is used to determine the data type of a variable or expression.

17. Error Handling in JavaScript: JavaScript uses try-catch blocks for handling errors and exceptions. The throw statement is used to throw custom exceptions.

18. Event-Driven Programming: Event-driven programming is a paradigm where the flow of the program is determined by events such as user actions, sensor outputs, or messages from other programs.

19. Purpose of "async" and "await" Keywords: The async keyword is used to define asynchronous functions, and the await keyword is used inside async functions to wait for a Promise to resolve before continuing.

20. Deep Copy vs. Shallow Copy:

    • A deep copy creates a new object and recursively copies all nested objects, ensuring that changes in the original object do not affect the copied one.
    • A shallow copy creates a new object and copies the values of the original object's properties, but not nested objects.

21. Memory Management in JavaScript: JavaScript uses automatic memory management (garbage collection) to allocate and deallocate memory. Developers don't manually allocate or deallocate memory.

22. Event Loop in JavaScript: The event loop is the mechanism that handles asynchronous operations in JavaScript. It continuously checks the message queue for events and executes them in a non-blocking manner.

23. Purpose of "map" Method: The map method is used to create a new array by applying a provided function to each element of the original array.

24. Promise in JavaScript: A Promise is an object representing the eventual completion or failure of an asynchronous operation, and its resulting value.

25. Handling Errors in Promises: Errors in Promises can be handled using the catch method or by attaching a catch block to the Promise chain.

26. Currying in JavaScript: Currying is a technique of transforming a function with multiple arguments into a sequence of functions with a single argument.

27. Purpose of "reduce" Method: The reduce method in JavaScript is used to reduce the elements of an array to a single value, applying a specified function from left to right.

28. Difference Between "null" and "undefined":

    • null is an intentional absence of any object value.
    • undefined is the default value of uninitialized variables or the value of missing properties in objects.

29. Types of Loops in JavaScript: JavaScript supports for, while, do-while, for...in, and for...of loops.

30. Difference Between "let," "const," and "var":

    • var is function-scoped and can be redeclared.
    • let and const are block-scoped, but let allows redeclaration, while const does not and requires an initial value.

31. Event Propagation in JavaScript: Event propagation refers to the order in which events are handled as they move through the DOM. It includes the capturing phase, target phase, and bubbling phase.

32. Manipulating the DOM: The DOM can be manipulated using methods like getElementById, createElement, appendChild, removeChild, setAttribute, and more.

33. localStorage and sessionStorage Objects: localStorage and sessionStorage are web storage objects that provide a way to store key/value pairs locally in the user's browser. localStorage persists even after the browser is closed, while sessionStorage is session-specific.

34. Handling Asynchronous Operations: Asynchronous operations in JavaScript can be handled using callbacks, Promises, async/await, and event listeners.

35. Purpose of "forEach" Method: The forEach method is used to iterate over elements of an

array, executing a provided function for each element.

36. Differences Between "let" and "var":

    • var is function-scoped, while let is block-scoped.
    • var can be hoisted to the top of its scope, while let is not.

37. Memoization in JavaScript: Memoization is a technique to optimize function performance by caching and reusing previously computed results for the same input.

38. Purpose of "splice" Method: The splice method in JavaScript is used to add or remove elements from an array at a specific index.

39. Generator Function: A generator function is a special type of function in JavaScript that allows you to control the execution flow using the yield keyword.

40. Variable Scoping in JavaScript: JavaScript uses function scope for variables declared with var and block scope for variables declared with let and const.

41. Purpose of "split" Method: The split method is used to split a string into an array of substrings based on a specified separator.

42. Deep Clone vs. Shallow Clone:

    • A deep clone creates a new object with copies of all nested objects, preventing any reference to the original.
    • A shallow clone creates a new object with references to the same nested objects as the original.

43. Event Delegation Pattern: Event delegation involves using a common ancestor to manage events for multiple elements, improving performance and simplifying event handling.

44. Differences Between "null" and "undefined":

    • null is an intentional absence of any object value.
    • undefined is the default value of uninitialized variables or the value of missing properties in objects.

45. Purpose of "arguments" Object: The arguments object in JavaScript contains an array-like list of the arguments passed to a function, enabling access to them within the function.

46. Ways to Define Methods in JavaScript Objects: Methods in JavaScript objects can be defined using shorthand notation, function expressions, or as properties with assigned functions.

47. Memoization and Its Benefits: Memoization is a technique to optimize function performance by caching and reusing previously computed results for the same input. It reduces redundant computations.

48. Difference Between "slice" and "splice":

    • slice returns a shallow copy of a portion of an array.
    • splice modifies an array by removing or replacing existing elements or adding new elements.

49. Purpose of "apply" and "call" Methods: Both apply and call are methods used to invoke a function with a specified this value and arguments. The difference is in how arguments are passed.

50. Event Loop and Asynchronous Operations: The event loop in JavaScript is a continuous process that manages the execution of code, handling asynchronous operations by placing callbacks in the message queue and executing them when the stack is empty. This ensures non-blocking behavior.