JavaScript Interview Questions and Answers for internship

1. What is JavaScript?

   • Answer: JavaScript is a high-level, interpreted programming language primarily used to make websites interactive. It's a dynamic, weakly-typed language that runs in web browsers and increasingly in server-side environments (Node.js).

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

   • Answer: `==` performs loose equality comparison, meaning it converts the operands to the same type before comparison. `===` performs strict equality comparison, requiring both the value and the type to be the same. For example, `1 == "1"` is true, but `1 === "1"` is false.

3. What are JavaScript data types?

   • Answer: JavaScript has several fundamental data types: Number (includes integers and floating-point numbers), String (text), Boolean (true or false), Null (intentional absence of value), Undefined (variable declared but not assigned), Symbol (unique values), BigInt (for arbitrarily large integers), and Object (collections of key-value pairs).

4. What is hoisting in JavaScript?

   • Answer: Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their scope before code execution. This doesn't apply to variable assignments, only declarations. While it can seem convenient, it's best to declare variables at the top of their scope for code clarity.

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

   • Answer: `var` declares a function-scoped or globally-scoped variable. `let` declares a block-scoped variable (within `{}`). `const` declares a block-scoped constant whose value cannot be reassigned after initialization. `let` and `const` are preferred over `var` in modern JavaScript.

6. Explain closures in JavaScript.

   • Answer: A closure is a function that has access to variables from its surrounding (enclosing) function's scope, even after the outer function has finished executing. This allows inner functions to "remember" variables from their parent function.

7. What are arrow functions?

   • Answer: Arrow functions provide a shorter syntax for writing functions. They implicitly return a value if the function body is a single expression. They also lexically bind `this`, meaning `this` refers to the surrounding scope, unlike regular functions.

8. What is the DOM (Document Object Model)?

   • Answer: The DOM is a programming interface for HTML and XML documents. It represents the page so that programs can change the document structure, style, and content. JavaScript manipulates the DOM to dynamically update web pages.

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

   • Answer: You can select elements using methods like `getElementById()`, `getElementsByClassName()`, `querySelector()` (for CSS selectors), and `querySelectorAll()` (for all matching elements).

10. Explain event handling in JavaScript.

    • Answer: Event handling allows JavaScript to respond to user actions or other events (like page loading). Events are attached to elements using methods like `addEventListener()` or inline event handlers (generally discouraged). The event handler function executes when the event occurs.

11. What is AJAX?

    • Answer: AJAX (Asynchronous JavaScript and XML) is a technique for updating parts of a web page without reloading the whole page. It uses XMLHttpRequest (or the newer Fetch API) to communicate asynchronously with a server.

12. 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 in AJAX requests to send and receive data from servers.

13. Explain promises in JavaScript.

    • Answer: Promises represent the eventual result of an asynchronous operation. They have three states: pending, fulfilled (success), and rejected (failure). They improve code readability and manage asynchronous operations more effectively than callbacks.

14. What is async/await?

    • Answer: `async/await` makes asynchronous code look and behave a bit more like synchronous code. `async` declares an asynchronous function, and `await` pauses execution until a Promise is resolved (or rejected).

15. What are some common JavaScript frameworks or libraries?

    • Answer: Popular choices include React, Angular, Vue.js, jQuery (older but still used), Node.js (server-side), and many others.

16. Explain the concept of prototype inheritance in JavaScript.

    • Answer: JavaScript uses prototype-based inheritance. Objects inherit properties and methods from their prototype. The prototype chain allows objects to access methods and properties from their prototype, and so on up the chain.

17. What is a JavaScript module?

    • Answer: A JavaScript module is a file containing JavaScript code that can be imported and used in other modules. This promotes code reusability and organization. Modern JavaScript uses `import` and `export` keywords for modules.

18. How do you handle errors in JavaScript?

    • Answer: Use `try...catch` blocks to handle exceptions. The `try` block contains code that might throw an error, and the `catch` block handles the error if one occurs. `finally` blocks can contain cleanup code that always runs.

19. What are some ways to debug JavaScript code?

    • Answer: Use your browser's developer tools (usually accessed by pressing F12), which include a debugger, console for logging messages, and network inspector. Also, use `console.log()` statements to print values for debugging.

20. Explain the difference between synchronous and asynchronous JavaScript.

    • Answer: Synchronous code executes line by line. Asynchronous code allows other code to run while waiting for an operation to complete (e.g., network request). Asynchronous code is essential for responsive web applications.

21. What is the difference between `null` and `undefined`?

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

22. What is the purpose of the `this` keyword in JavaScript?

    • Answer: The `this` keyword refers to the current context or the object that a method is called upon. Its value can vary depending on how the function is called (e.g., as a method of an object, or as a standalone function).

23. How do you create an object in JavaScript?

    • Answer: You can create objects using object literals `{}`, object constructors, or classes (using ES6 classes).

24. What are JavaScript methods?

    • Answer: Methods are functions that are associated with an object. They operate on the object's data.

25. What is a callback function?

    • Answer: A callback function is a function passed as an argument to another function, which is then invoked inside the outer function at a later time.

26. What are immediately invoked function expressions (IIFEs)?

    • Answer: IIFEs are functions that are defined and immediately executed. They are often used to create private scopes for variables and functions.

27. Explain the concept of scope in JavaScript.

    • Answer: Scope determines the accessibility of variables. Variables declared within a function have local scope, while variables declared outside any function have global scope. Block scope is defined by curly braces `{}`.

28. What is the difference between `for` and `for...of` loops?

    • Answer: `for` loops iterate over a sequence using a counter. `for...of` loops iterate over iterable objects (like arrays, strings, maps) directly, accessing each element.

29. What is the difference between `forEach` and `map` array methods?

    • Answer: `forEach` iterates over an array and executes a function for each element. `map` creates a new array with the results of calling a function on each element of the original array.

30. What is the `filter` array method?

    • Answer: `filter` creates a new array containing only the elements that pass a provided test function.

31. What is the `reduce` array method?

    • Answer: `reduce` applies a function cumulatively to the array elements, reducing them to a single value.

32. What is event bubbling?

    • Answer: Event bubbling is the order in which events are handled when an element is nested within other elements. The event is handled first by the innermost element and then bubbles up to its ancestors.

33. What is event capturing?

    • Answer: Event capturing is the opposite of event bubbling. The event is handled first by the outermost ancestor and then propagates down to the target element.

34. How do you prevent default behavior of an event?

    • Answer: Use the `preventDefault()` method of the event object.

35. What is a regular expression?

    • Answer: A regular expression is a pattern that can be used to search for and manipulate text.

36. How do you create and use a regular expression in JavaScript?

    • Answer: Use the `/pattern/flags` literal notation or the `RegExp` constructor. Methods like `test()`, `match()`, `replace()` are used to work with regular expressions.

37. What is the difference between `let` and `var` in terms of scope?

    • Answer: `var` has function scope, while `let` has block scope.

38. What is a JavaScript class?

    • Answer: A class is a blueprint for creating objects. It defines properties and methods that objects of that class will have.

39. How do you create and use a JavaScript class?

    • Answer: Use the `class` keyword to define a class. Use the `new` keyword to create an instance of a class.

40. What is a constructor in a JavaScript class?

    • Answer: The constructor is a special method that is called when a new object is created from a class. It is used to initialize the object's properties.

41. What are static methods in a JavaScript class?

    • Answer: Static methods are methods that belong to the class itself, not to instances of the class. They are called using the class name.

42. What is inheritance in JavaScript classes?

    • Answer: Inheritance allows a class to inherit properties and methods from another class (its parent class). The `extends` keyword is used for inheritance.

43. What is polymorphism in JavaScript?

    • Answer: Polymorphism allows objects of different classes to be treated as objects of a common type. This is often achieved through method overriding.

44. What is the difference between `==` and `===`?

    • Answer: `==` performs loose equality comparison (type coercion), while `===` performs strict equality comparison (no type coercion).

45. What is truthy and falsy in JavaScript?

    • Answer: Truthy values are values that evaluate to true in a boolean context. Falsy values are values that evaluate to false in a boolean context (e.g., `false`, `0`, `""`, `null`, `undefined`, `NaN`).

46. How do you convert a string to a number in JavaScript?

    • Answer: Use `parseInt()` for integers and `parseFloat()` for floating-point numbers.

47. How do you convert a number to a string in JavaScript?

    • Answer: Use the `toString()` method or string concatenation.

48. What is the use of the `typeof` operator?

    • Answer: The `typeof` operator returns the type of a variable or value.

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

    • Answer: Use `Array.isArray()`.

50. What is the purpose of the `Object.keys()` method?

    • Answer: It returns an array of an object's own enumerable property names.

51. What is the purpose of the `Object.values()` method?

    • Answer: It returns an array of an object's own enumerable property values.

52. What is the purpose of the `Object.entries()` method?

    • Answer: It returns an array of an object's own enumerable property [key, value] pairs.

53. How do you create a new array from an existing array?

    • Answer: Use the `slice()` method to create a shallow copy, or the spread syntax (`...`) for a shallow copy.

54. Explain the difference between shallow copy and deep copy.

    • Answer: A shallow copy creates a new object but copies only the references to nested objects. A deep copy creates a completely independent copy of the original object and all its nested objects.

55. How do you create a deep copy of an object in JavaScript?

    • Answer: Use `JSON.parse(JSON.stringify(object))` (for simple objects) or a library like Lodash's `cloneDeep()` for more complex objects.

56. What are the different ways to empty an array in JavaScript?

    • Answer: `arr.length = 0`, `arr.splice(0, arr.length)`, `arr = []` (reassigns the variable).

57. What is the difference between `map` and `forEach`?

    • Answer: `map` returns a new array with transformed elements, while `forEach` only iterates and does not return anything.

58. What is a ternary operator?

    • Answer: A ternary operator is a shorthand for an `if...else` statement: `condition ? valueIfTrue : valueIfFalse`.

59. What are some best practices for writing clean and maintainable JavaScript code?

    • Answer: Use consistent indentation, meaningful variable names, comments, modular code, and follow a coding style guide.

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