JavaScript Interview Questions and Answers

1. What is JavaScript?

   • Answer: JavaScript is a high-level, interpreted programming language that conforms to the ECMAScript specification. It's primarily used for creating interactive and dynamic web pages, but also finds applications in server-side programming (Node.js), mobile app development (React Native), and more.

2. What are the data types in JavaScript?

   • Answer: JavaScript has dynamic typing. Common data types include: Number (includes integers and floating-point numbers), String, Boolean (true or false), null, undefined, Symbol (unique values), BigInt (for arbitrarily large integers), and Object (collections of key-value pairs).

3. Explain the difference between == and === in JavaScript.

   • Answer: `==` performs loose equality comparison, converting types before comparing values. `===` performs strict equality comparison, requiring both value and type to be the same. For example, `1 == "1"` is true, but `1 === "1"` is false.

4. What is hoisting in JavaScript?

   • Answer: Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their scope before code execution. However, only the *declaration* is hoisted, not the *initialization*. This means a variable will be undefined before its declaration line.

5. What are closures in JavaScript?

   • Answer: A closure is a function that has access to variables from its surrounding scope, even after that scope has finished executing. This allows inner functions to "remember" and access variables from their parent functions.

6. Explain the concept of "this" in JavaScript.

   • Answer: The `this` keyword refers to the object that is executing the current function. Its value depends on how the function is called (e.g., as a method of an object, in a function call, or using `call` or `apply`). It can be confusing and requires careful consideration.

7. What are prototypes in JavaScript?

   • Answer: Prototypes are a mechanism for inheritance in JavaScript. Every object has a prototype, which is another object that it inherits properties and methods from. This creates a prototype chain.

8. What is the difference between `let`, `const`, and `var`?

   • Answer: `var` is function-scoped (or globally scoped if not inside a function). `let` and `const` are block-scoped (within the nearest enclosing block, e.g., `{}`). `const` declares a constant value, which cannot be reassigned after initialization. `let` allows reassignment.

9. Explain how to create an object in JavaScript.

   • Answer: Objects can be created using object literals (`{ key: value, ... }`), the `new Object()` constructor, or classes (ES6 and later).

10. What are arrow functions in JavaScript?

    • Answer: Arrow functions provide a concise syntax for writing functions. They lexically bind `this`, meaning `this` refers to the surrounding scope rather than the object the function is called upon.

11. What is the purpose of `async/await`?

    • Answer: `async/await` makes asynchronous code easier to read and write. `async` declares an asynchronous function that implicitly returns a Promise. `await` pauses execution until a Promise resolves.

12. Explain Promises in JavaScript.

    • Answer: Promises represent the eventual result of an asynchronous operation. They have three states: pending, fulfilled (resolved), and rejected. They allow for cleaner handling of asynchronous operations than callbacks.

13. What are JavaScript frameworks and libraries? Give some examples.

    • Answer: Frameworks (like React, Angular, Vue.js) provide a structure and guidelines for building applications. Libraries (like jQuery, Lodash) offer reusable functions and components. Frameworks often dictate how you structure your code, while libraries are used more flexibly.

14. What is DOM manipulation?

    • Answer: DOM (Document Object Model) manipulation is the process of changing the structure, style, or content of a web page's HTML using JavaScript. It allows for dynamic updates to the page without requiring a full reload.

15. Explain event handling in JavaScript.

    • Answer: Event handling involves responding to user interactions (like clicks, mouseovers, key presses) or other events on a web page. This is typically done by attaching event listeners to HTML elements.

16. What is JSON?

    • Answer: JSON (JavaScript Object Notation) is a lightweight data-interchange format. It's easy for humans to read and write, and easy for machines to parse and generate. It's commonly used for transmitting data between a server and a web client.

17. How do you handle errors in JavaScript?

    • Answer: Errors can be handled using `try...catch` blocks. The `try` block contains code that might throw an error, and the `catch` block handles any errors that occur.

18. What are some common JavaScript design patterns?

    • Answer: Common patterns include Module, Singleton, Factory, Observer, and MVC (Model-View-Controller).

19. Explain the concept of AJAX.

    • Answer: AJAX (Asynchronous JavaScript and XML) allows web pages to update content asynchronously, without requiring a full page reload. It's often used to fetch data from a server in the background.

20. What is the difference between local storage and session storage?

    • Answer: Both are ways to store data in the user's browser. Local storage persists data even after the browser is closed, while session storage only keeps data for the duration of the browser session.

21. How do you debug JavaScript code?

    • Answer: Techniques include using the browser's developer tools (console logging, breakpoints, stepping through code), using a debugger, and adding `console.log` statements to check variable values.

22. What are some common JavaScript libraries for DOM manipulation?

    • Answer: jQuery is a popular choice, although its usage is declining with the rise of modern frameworks. Many frameworks (React, Angular, Vue) handle DOM manipulation internally.

23. What is a callback function?

    • Answer: A callback function is a function passed as an argument to another function. It's executed after the completion of the outer function, often used for handling asynchronous operations.

24. Explain the concept of event bubbling.

    • Answer: When an event occurs on an element nested within other elements, the event "bubbles" up the DOM tree, triggering event listeners on parent elements as well. Event capturing is the opposite.

25. What is the purpose of the `map()` method?

    • Answer: The `map()` method creates a new array by applying a provided function to each element in an existing array.

26. What is the purpose of the `filter()` method?

    • Answer: The `filter()` method creates a new array containing only the elements from an existing array that pass a certain condition (specified by a provided function).

27. What is the purpose of the `reduce()` method?

    • Answer: The `reduce()` method applies a function cumulatively to the array elements to reduce the array to a single value.

28. What are some ways to handle asynchronous operations in JavaScript besides Promises and async/await?

    • Answer: Callbacks are a more traditional method, and there are also libraries like Async that provide more advanced tools for managing asynchronous code.

29. How do you create a self-executing anonymous function?

    • Answer: By wrapping the function definition in parentheses and immediately invoking it with another set of parentheses: `(function(){ ... })();`

30. Explain the difference between synchronous and asynchronous operations.

    • Answer: Synchronous operations execute one after another in a sequential order. Asynchronous operations can execute concurrently, allowing the program to continue running while waiting for the result of an operation (e.g., network request).

31. What is a JavaScript module?

    • Answer: A JavaScript module is a file containing JavaScript code that can be imported and used by other modules. This promotes code organization and reusability.

32. Explain how to import and export modules in JavaScript.

    • Answer: `export` is used in a module to make variables, functions, or classes available for import. `import` is used in another module to bring in those exported items. Various syntaxes exist depending on the module system used (e.g., ES modules, CommonJS).

33. What is the use of the `setTimeout()` function?

    • Answer: `setTimeout()` executes a provided function after a specified delay (in milliseconds).

34. What is the use of the `setInterval()` function?

    • Answer: `setInterval()` repeatedly executes a function at a specified interval (in milliseconds).

35. How can you prevent default behavior of an event?

    • Answer: By calling `event.preventDefault()` within the event handler function.

36. What is the difference between innerHTML and textContent?

    • Answer: `innerHTML` sets or gets the HTML content of an element, including any HTML tags. `textContent` sets or gets the plain text content of an element, ignoring any HTML tags.

37. Explain the concept of event delegation.

    • Answer: Event delegation involves attaching an event listener to a parent element, rather than individual child elements. The listener checks the target of the event to determine which child element triggered it. This is more efficient for handling events on many similar elements.

38. How do you create a regular expression in JavaScript?

    • Answer: Using forward slashes to enclose the pattern: `/pattern/flags` (flags like `g` for global, `i` for case-insensitive).

39. Explain some common regular expression methods in JavaScript.

    • Answer: `test()` to check if a string matches a pattern, `match()` to find all matches, `replace()` to replace matches with another string.

40. What is a JavaScript class?

    • Answer: A blueprint for creating objects. It defines properties (data) and methods (functions) that objects of that class will have.

41. Explain the keywords `extends` and `super` in JavaScript classes.

    • Answer: `extends` is used for inheritance – to create a new class that inherits properties and methods from an existing class. `super()` is used in the constructor of the child class to call the constructor of the parent class.

42. What are getters and setters in JavaScript classes?

    • Answer: Getters provide a way to access the value of a property, and setters provide a way to set the value, often with additional logic (e.g., validation).

43. How do you handle multiple asynchronous operations concurrently?

    • Answer: Using `Promise.all()` to wait for all Promises in an array to resolve, or using `async/await` with parallel `await` calls.

44. What is the difference between null and undefined?

    • Answer: `null` is an assignment value that represents the intentional absence of a value. `undefined` indicates that a variable has been declared but has not been assigned a value.

45. What is strict mode in JavaScript?

    • Answer: Strict mode (`"use strict";`) enforces stricter parsing and error handling, preventing some common coding errors and improving code quality. It introduces several restrictions and changes to how JavaScript behaves.

46. Explain how to create and use a JavaScript Map.

    • Answer: A `Map` is a collection of key-value pairs where keys can be of any data type. It's created using `new Map()` and allows efficient key-based access to values. It offers methods like `set()`, `get()`, `has()`, `delete()`.

47. Explain how to create and use a JavaScript Set.

    • Answer: A `Set` is a collection of unique values. It's created using `new Set()` and offers methods like `add()`, `has()`, `delete()`, `clear()`. Useful for removing duplicates from an array.

48. What is the difference between a for...of loop and a for...in loop?

    • Answer: `for...of` iterates over iterable objects (like arrays and strings), providing the values directly. `for...in` iterates over the enumerable properties of an object, providing the keys.

49. How do you check if a variable is an array?

    • Answer: Using `Array.isArray(variable)`.

50. How do you empty an array?

    • Answer: Several ways: `array.length = 0;`, `array.splice(0, array.length);`, or reassigning the array to an empty array: `array = [];`.

51. How do you copy an array in JavaScript?

    • Answer: Use the spread syntax (`[...array]`), `array.slice()`, or `Array.from(array)` for shallow copies. For deep copies (copying nested objects), consider using `JSON.parse(JSON.stringify(array))` or a library like Lodash's `cloneDeep()`.

52. What is a Web Worker?

    • Answer: A Web Worker allows you to run JavaScript code in a separate thread, preventing blocking of the main thread (important for performance, especially with long-running tasks).

53. What are some common methods for making HTTP requests in JavaScript?

    • Answer: The `fetch` API is a modern approach. Older methods include the `XMLHttpRequest` object.

54. What are the differences between `==`, `===`, and `Object.is()`?

    • Answer: `==` is loose equality (type coercion), `===` is strict equality (no type coercion). `Object.is()` is a stricter equality comparison than `===`, handling `NaN` and `-0` differently.

55. Explain the concept of immutability in JavaScript.

    • Answer: Immutability means that once an object is created, its state cannot be modified. This can be beneficial for simplifying state management and improving application predictability.

56. What are some ways to achieve immutability in JavaScript?

    • Answer: Using the spread operator to create copies before modifying, using libraries like Immer, or using functional programming techniques.

57. What is the difference between shallow copy and deep copy?

    • Answer: A shallow copy creates a new object, but the nested objects within are still references to the original objects. A deep copy creates a completely independent copy of the original object, including all nested objects.

58. Explain how to use the `debounce` and `throttle` techniques.

    • Answer: Debounce delays the execution of a function until a certain amount of time has passed since the last call. Throttle limits the rate at which a function can be executed, ensuring a maximum number of executions per time period. These are used to improve performance with events like window resize or scroll.

59. What is a higher-order function?

    • Answer: A higher-order function is a function that takes one or more functions as arguments or returns a function as its result.

60. What is currying?

    • Answer: Currying transforms a function that takes multiple arguments into a sequence of functions that each take a single argument.

61. What is function composition?

    • Answer: Function composition involves chaining multiple functions together, where the output of one function becomes the input of the next.

62. How do you handle CORS issues?

    • Answer: CORS (Cross-Origin Resource Sharing) restrictions prevent requests from one domain to another. Solutions include configuring the server to allow CORS, using a proxy server, or using JSONP (for GET requests).

63. Explain how to work with local storage and session storage in JavaScript.

    • Answer: Use `localStorage.setItem()`, `localStorage.getItem()`, `localStorage.removeItem()`, and similar methods for sessionStorage. Remember to handle potential errors and consider security implications.

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