1. What is JavaScript?

JavaScript is a high-level, interpreted programming language primarily used for adding interactivity and behavior to web pages.

2. What are the data types in JavaScript? JavaScript has seven primitive data types: undefined, null, boolean, number, string, symbol, and bigint, along with an object data type.

3. Explain the difference between null and undefined. null represents an intentionally assigned absence of any object value, while undefined indicates that a variable has been declared but has not been assigned a value.

4. How do you declare variables in JavaScript? You can declare variables using var, let, or const. let and const were introduced in ES6 (ECMAScript 2015) and have block scope.

5. What is hoisting in JavaScript? Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during compilation, making them accessible before their actual declaration in code.

6. What are closures in JavaScript? Closures are functions that have access to variables from their outer (enclosing) function's scope, even after the outer function has completed execution.

7. What is the 'this' keyword in JavaScript? this refers to the current object or context in which a function is executed. Its value can change depending on how a function is called.

8. Explain callback functions in JavaScript. Callback functions are functions passed as arguments to other functions and are executed at a later time, often in response to an event or asynchronous operation.

9. What is a promise in JavaScript? A promise is an object representing the eventual completion or failure of an asynchronous operation. It allows you to handle asynchronous code in a more readable and maintainable way.

10. What is an async/await in JavaScript? Async/await is a modern syntax for handling asynchronous operations. The async keyword is used to declare an asynchronous function, and await is used to pause the execution of a function until a promise is resolved.

11. What is the difference between '=='' and '===' in JavaScript? == compares values for equality with type coercion, while === compares both values and types for strict equality.

12. Explain the event delegation in JavaScript. Event delegation is a technique where you attach a single event listener to a parent element to handle events on its child elements. It improves performance and reduces memory usage.

13. What is the purpose of the bind() method in JavaScript? The bind() method creates a new function with a specified this value and partially applies arguments to it. It's often used for setting the context of a function.

14. What is the purpose of the map() method in JavaScript? The map() method is used to create a new array by applying a provided function to each element of an existing array.

15. What is the purpose of the filter() method in JavaScript? The filter() method creates a new array with all elements that pass a test specified by a provided function.

16. How do you handle errors in JavaScript? Errors can be handled using try...catch blocks to gracefully handle exceptions and prevent them from crashing the program.

17. Explain the same-origin policy in JavaScript. The same-origin policy is a security feature that restricts web pages from making requests to domains different from the one that served the web page. This policy helps prevent cross-site scripting (XSS) attacks.

18. What is a cookie in JavaScript? A cookie is a small piece of data stored on a user's computer by a web browser. Cookies are often used for tracking user sessions and preferences.

19. What is the difference between 'null' and 'undefined'? null is an intentionally assigned absence of any object value, while undefined is a variable that has been declared but not assigned a value.

20. Explain the purpose of the localStorage and sessionStorage objects. localStorage and sessionStorage are used to store key-value pairs in a web browser. localStorage data persists across browser sessions, while sessionStorage data is only available during the current session.

21. What is the difference between let, const, and var for variable declaration? let and const have block scope, while var has function scope. let allows reassignment, const does not, and var has no restrictions.

22. What is the event loop in JavaScript, and how does it work? The event loop is a core part of JavaScript's concurrency model. It continuously checks the call stack for functions to execute and the message queue for events to process, allowing non-blocking operations.

23. What is the purpose of the addEventListener method in JavaScript? addEventListener is used to attach event handlers to DOM elements, enabling them to respond to user actions like clicks and keypresses.

24. Explain the difference between localStorage and sessionStorage. localStorage stores data with no expiration time, while sessionStorage stores data for a single session, clearing it when the session ends.

25. What is JSON, and how is it related to JavaScript? JSON (JavaScript Object Notation) is a lightweight data-interchange format used for data serialization. It is closely related to JavaScript because it shares syntax and is often used to exchange data between a server and a web application.

26. What are arrow functions in JavaScript? Arrow functions are a concise way to write functions in JavaScript, introduced in ES6. They have a shorter syntax and inherit the this value from their surrounding code.

27. What is the purpose of the fetch() API in JavaScript? The fetch() API is used for making network requests and fetching resources, typically for making HTTP requests to a server.

28. Explain the concept of the event bubbling and event capturing in JavaScript. Event bubbling is the default behavior in which an event starts from the target element and bubbles up through its ancestors. Event capturing, on the other hand, starts from the top-level ancestor and goes down to the target element.

29. What is a closure in JavaScript, and why is it useful? A closure is a function that has access to variables from its outer (enclosing) function's scope. Closures are useful for data encapsulation and creating private variables.

30. What is the purpose of the Object.keys() method in JavaScript? The Object.keys() method returns an array of a given object's enumerable property names, allowing you to iterate over an object's keys.

31. Explain the concept of debouncing and throttling in JavaScript. Debouncing and throttling are techniques used to limit the frequency of function calls. Debouncing delays the execution of a function until a pause in input, while throttling limits the function's execution to a fixed interval.

32. How can you deep-clone an object in JavaScript? You can deep-clone an object using various methods, such as the JSON.parse(JSON.stringify(obj)) approach or third-party libraries like lodash.

33. What is the purpose of the JavaScript map() function? The map() function is used to create a new array by applying a provided function to each element of an existing array. It doesn't modify the original array.

34. What is a closure and how is it useful in JavaScript? A closure is a function that has access to variables from its outer (enclosing) function's scope, even after the outer function has finished executing. Closures are useful for creating private variables and maintaining state.

35. Explain the difference between null and undefined in JavaScript. null represents the intentional absence of any object value, while undefined is a variable that has been declared but has not been assigned a value.

36. What are template literals in JavaScript? Template literals, also known as template strings, are string literals that allow you to embed expressions and variables inside a string using backticks ( ) as delimiters. They support multi-line strings and string interpolation.

37. How do you check the data type of a variable in JavaScript? You can use the typeof operator to check the data type of a variable. For example, typeof x will return a string indicating the type of x.

38. What is the purpose of the JavaScript reduce() method? The reduce() method is used to reduce an array to a single value by applying a specified function to each element. It accumulates the results and returns the final value.

39. What is the difference between 'async' and 'defer' attributes in a script tag? The async attribute specifies that the script should be executed asynchronously, while the defer attribute indicates that the script should be executed after the document has been parsed, but before the DOMContentLoaded event.

40. Explain the use of the Set object in JavaScript. A Set is a collection of unique values, which means it can store only distinct values. It's often used to remove duplicates from an array or check for the existence of unique values.

41. How does prototypal inheritance work in JavaScript? In JavaScript, objects inherit properties and methods from their prototype objects. Each object has a prototype chain, which allows for inheritance and property lookup.

42. What is the difference between window.onload and the DOMContentLoaded event? window.onload waits for all assets (images, stylesheets, etc.) to be fully loaded before firing, while the DOMContentLoaded event triggers when the HTML document has been completely parsed, which may occur before all assets are loaded.

43. How can you create and manipulate elements in the DOM using JavaScript? You can create and manipulate DOM elements using methods like document.createElement(), element.appendChild(), element.innerHTML, and many more.

44. Explain the purpose of the JavaScript bind() method. The bind() method is used to create a new function with a specific this value and optional arguments. It allows you to set the context of a function, ensuring that this refers to a specific object.

45. What are arrow functions in JavaScript, and when should you use them? Arrow functions are a concise way to write functions in JavaScript, introduced in ES6. They have a shorter syntax and inherit the this value from their enclosing scope. Arrow functions are especially useful for writing short, one-liner functions.

46. What is the Event Loop in JavaScript, and why is it important for asynchronous operations? The Event Loop is a critical part of JavaScript's concurrency model. It manages the execution of code and handles asynchronous operations, ensuring that non-blocking tasks, such as handling events and making network requests, can be executed efficiently.

47. How do you handle exceptions and errors in JavaScript? You can handle exceptions and errors using try...catch blocks. When an error occurs within a try block, control is transferred to the catch block, allowing you to handle the error gracefully.

48. Explain the concept of lexical scope in JavaScript. Lexical scope refers to the way variable scope is determined by the physical placement of variables within the code. Inner functions have access to variables in their outer (enclosing) functions.

49. What is the purpose of the JavaScript Promise object, and how does it work? The Promise object represents the eventual completion (or failure) of an asynchronous operation and provides a more structured way to handle asynchronous code than callbacks. It has states such as pending, fulfilled, and rejected.

50. What is the difference between a function declaration and a function expression in JavaScript? A function declaration is hoisted and can be used before its declaration in the code, while a function expression is not hoisted and can only be used after its declaration.