ES6 Interview Questions and Answers for freshers

1. What is ES6 (ECMAScript 2015)?

   • Answer: ES6, also known as ECMAScript 2015, is a major update to the JavaScript language. It introduced many new features to improve code readability, maintainability, and performance.

2. Explain `let` and `const`.

   • Answer: `let` declares a block-scoped variable, meaning it's only accessible within the block it's defined in. `const` declares a block-scoped constant. Once assigned a value, it cannot be reassigned. However, if it's an object or array, its properties or elements can be modified.

3. What are arrow functions?

   • Answer: Arrow functions provide a shorter syntax for writing functions. They lexically bind `this`, meaning `this` refers to its surrounding scope, unlike regular functions.

4. Explain template literals.

   • Answer: Template literals use backticks (`) to define strings. They allow embedded expressions using `${expression}`. They also support multiline strings.

5. What are default parameters?

   • Answer: Default parameters allow you to provide default values for function parameters if no value is passed during the function call.

6. Explain the spread syntax (...).

   • Answer: The spread syntax allows you to expand iterable objects like arrays and strings into individual elements. It's also used to create copies of arrays and objects.

7. What are rest parameters?

   • Answer: Rest parameters allow a function to accept an indefinite number of arguments as an array.

8. Describe destructuring assignment.

   • Answer: Destructuring assignment allows you to unpack values from arrays or objects into distinct variables.

9. Explain classes in ES6.

   • Answer: ES6 classes provide a syntactic sugar over prototypal inheritance. They offer a cleaner way to define classes and their methods.

10. What are modules in ES6?

    • Answer: ES6 modules allow you to organize code into reusable modules, improving code organization and maintainability. They use `import` and `export` keywords.

11. Explain `import` and `export` statements.

    • Answer: `export` is used in a module to make variables, functions, or classes available for other modules to use. `import` is used in another module to access those exported elements.

12. What are Promises?

    • Answer: Promises represent the eventual result of an asynchronous operation. They handle asynchronous code more elegantly than callbacks.

13. Explain `Promise.all`.

    • Answer: `Promise.all` takes an array of promises and returns a single promise that resolves when all the input promises have resolved, or rejects if any of them reject.

14. Explain `Promise.race`.

    • Answer: `Promise.race` takes an array of promises and returns a single promise that resolves or rejects as soon as one of the input promises resolves or rejects.

15. What are Sets?

    • Answer: Sets are collections of unique values. They provide methods for adding, deleting, and checking for the presence of elements.

16. What are Maps?

    • Answer: Maps are collections of key-value pairs, similar to objects, but keys can be of any data type, not just strings.

17. Explain WeakSets and WeakMaps.

    • Answer: WeakSets and WeakMaps are similar to Sets and Maps, but their keys (for WeakMaps) or values (for WeakSets) are held weakly. This means garbage collection can remove them even if they're referenced in the WeakSet or WeakMap, preventing memory leaks.

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

    • Answer: `==` performs loose equality comparison, while `===` performs strict equality comparison. Strict equality checks both value and type.

19. Explain `for...of` and `for...in` loops.

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

20. What is Symbol?

    • Answer: Symbols are unique and immutable values. They can be used as keys in objects to create private properties.

21. Explain `Object.assign`.

    • Answer: `Object.assign` copies the values of all enumerable own properties from one or more source objects to a target object.

22. What is `Object.keys`, `Object.values`, `Object.entries`?

    • Answer: `Object.keys` returns an array of an object's own enumerable property keys. `Object.values` returns an array of an object's own enumerable property values. `Object.entries` returns an array of an object's own enumerable property key-value pairs as arrays.

23. Explain the concept of `this` in ES6.

    • Answer: The value of `this` depends on how a function is called. In arrow functions, `this` lexically binds to its surrounding scope. In regular functions, it can vary depending on the calling context.

24. How do you handle errors in ES6 using `try...catch`?

    • Answer: The `try...catch` statement allows you to handle exceptions gracefully. Code that might throw an error is placed in the `try` block, and the `catch` block handles any errors that occur.

25. What are generators in ES6?

    • Answer: Generators are functions that can be paused and resumed. They use the `function*` syntax and the `yield` keyword to pause execution and return a value.

26. Explain iterators in ES6.

    • Answer: Iterators are objects that define a sequence and a method to access the next element in the sequence. They are used with `for...of` loops.

27. What is the difference between `Array.from` and `Array.of`?

    • Answer: `Array.from` creates a new array from an array-like object or iterable. `Array.of` creates a new array with the given arguments as elements.

28. Explain `Array.prototype.find` and `Array.prototype.findIndex`.

    • Answer: `Array.prototype.find` returns the first element in an array that satisfies a provided testing function. `Array.prototype.findIndex` returns the index of the first element that satisfies a provided testing function.

29. What is `Array.prototype.includes`?

    • Answer: `Array.prototype.includes` determines whether an array includes a certain value among its entries, returning `true` or `false` as appropriate.

30. Explain `Array.prototype.map`, `Array.prototype.filter`, and `Array.prototype.reduce`.

    • Answer: `map` transforms each element of an array using a provided function. `filter` creates a new array with elements that pass a provided test. `reduce` applies a function cumulatively to each element to reduce the array to a single value.

31. What are Proxies in ES6?

    • Answer: Proxies allow you to intercept and customize operations performed on an object. You can intercept property access, assignments, function calls, etc.

32. What are Reflect APIs?

    • Answer: Reflect APIs provide methods to perform operations on objects and arrays, often mirroring the functionality of Proxy handlers.

33. How to create a shallow copy of an object?

    • Answer: You can use the spread syntax (`...`) or `Object.assign()` to create a shallow copy of an object. Note that this only copies the top-level properties; nested objects will still be references.

34. How to create a deep copy of an object?

    • Answer: You'll need a recursive function or a library like Lodash's `cloneDeep` to create a deep copy, ensuring that nested objects are also copied independently.

35. What are tagged template literals?

    • Answer: Tagged template literals allow you to pass template literals to a function, giving you the ability to pre-process the string before it's used.

36. Explain the concept of asynchronous programming in ES6.

    • Answer: Asynchronous programming allows you to handle long-running operations without blocking the main thread, enhancing responsiveness. Promises and async/await are key mechanisms.

37. Explain `async` and `await`.

    • Answer: `async` declares an asynchronous function that implicitly returns a Promise. `await` pauses execution within an async function until a Promise resolves.

38. How to handle asynchronous errors with `async`/`await`?

    • Answer: You can use a `try...catch` block within an `async` function to handle errors that might occur during `await` operations.

39. What are some best practices for using ES6 features?

    • Answer: Use `const` for constants, `let` for variables that change, and avoid using `var`. Utilize arrow functions appropriately. Write modular code using ES6 modules. Use Promises or async/await for asynchronous operations.

40. Explain the difference between a function declaration and a function expression.

    • Answer: Function declarations are hoisted, meaning they are accessible before their declaration in the code. Function expressions are not hoisted.

41. What is the difference between `Object.freeze` and `Object.seal`?

    • Answer: `Object.freeze` prevents any modifications to an object's properties (adding, deleting, or changing). `Object.seal` prevents adding or deleting properties but allows changing existing properties.

42. What is a higher-order function?

    • Answer: A higher-order function is a function that takes another function as an argument or returns a function as its result.

43. Explain the concept of closures in JavaScript.

    • Answer: A closure is a function that has access to variables from its surrounding scope (lexical environment) even after that scope has finished executing.

44. What is the role of the `new` keyword when creating objects?

    • Answer: The `new` keyword creates a new object, sets its prototype to the constructor's prototype, and binds `this` within the constructor function to the newly created object.

45. How does prototypal inheritance work in JavaScript?

    • Answer: Prototypal inheritance allows objects to inherit properties and methods from other objects. Each object has a prototype, and when a property is accessed on an object, the prototype chain is searched until the property is found.

46. What are some common use cases for Promises?

    • Answer: Handling asynchronous operations like AJAX requests, fetching data from APIs, and working with timers.

47. What are some common use cases for async/await?

    • Answer: Improving the readability and maintainability of asynchronous code, particularly when dealing with multiple promises.

48. Explain the concept of event loop in JavaScript.

    • Answer: The event loop is a mechanism that processes events and callbacks in JavaScript's single-threaded environment, enabling asynchronous behavior.

49. What is the difference between synchronous and asynchronous operations?

    • Answer: Synchronous operations are executed sequentially, blocking the execution of subsequent code until they complete. Asynchronous operations are executed concurrently without blocking the main thread.

50. How can you improve the performance of your JavaScript code?

    • Answer: Minimize DOM manipulations, optimize loops, use efficient data structures, avoid unnecessary calculations, and utilize caching mechanisms.

51. What are some common debugging techniques for JavaScript code?

    • Answer: Using browser developer tools (debugger, console), setting breakpoints, using `console.log` statements, and using linters and code analysis tools.

52. What are some common JavaScript frameworks or libraries that utilize ES6 features?

    • Answer: React, Angular, Vue.js, Node.js, and many others heavily rely on and leverage ES6 features.

53. Explain the concept of callback functions in JavaScript.

    • Answer: Callback functions are functions passed as arguments to other functions, typically used to handle asynchronous operations or events.

54. What are some common security considerations when working with JavaScript?

    • Answer: Input validation, output encoding, preventing cross-site scripting (XSS), and using secure coding practices to avoid vulnerabilities.

55. Explain how to use the `filter` method to create a new array containing only even numbers from an existing array.

    • Answer: `const evenNumbers = originalArray.filter(number => number % 2 === 0);`

56. How would you use the `map` method to double each number in an array?

    • Answer: `const doubledNumbers = originalArray.map(number => number * 2);`

57. How can you use the `reduce` method to sum all the numbers in an array?

    • Answer: `const sum = originalArray.reduce((accumulator, currentValue) => accumulator + currentValue, 0);`

58. Write a simple example using `async` and `await` to fetch data from an API.

    • Answer: (Requires a fetch API example, which is too lengthy for this format. A conceptual answer would suffice here: The answer would involve using `fetch()` within an `async` function and using `await` to wait for the promise to resolve before processing the data.)

59. Explain how you would handle a rejected promise using `.catch()`.

    • Answer: A `.catch()` block is added to the end of the promise chain to handle rejection. The `.catch()` block takes a function that accepts the reason for rejection as an argument.

60. Describe a scenario where you would use a `Set` instead of an array.

    • Answer: When you need to store a collection of unique values and don't care about the order. Example: Tracking unique user IDs.

61. When would you prefer using a `Map` over a plain JavaScript object?

    • Answer: When you need to use non-string keys (like numbers, objects, or symbols).

62. What is the purpose of the `finally` block in a `try...catch` statement?

    • Answer: The `finally` block always executes, regardless of whether an error occurred or not, usually for cleanup tasks.

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

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

64. Explain the concept of hoisting in JavaScript.

    • Answer: Hoisting is a JavaScript mechanism where declarations of variables and functions are moved to the top of their scope before code execution.

65. What is the difference between a class method and a static method?

    • Answer: Class methods are called on instances of the class. Static methods are called on the class itself.

66. Explain the concept of inheritance in the context of ES6 classes.

    • Answer: Inheritance allows a class to inherit properties and methods from a parent class using the `extends` keyword. It promotes code reuse and establishes an "is-a" relationship.

67. How can you prevent method overriding in ES6 classes?

    • Answer: By using the `#` symbol to create private methods, preventing access or overriding from subclasses.

68. Explain how to implement polymorphism in JavaScript using ES6 classes.

    • Answer: Polymorphism allows objects of different classes to respond to the same method call in their own specific way. This is achieved through method overriding in subclasses.

69. Describe how you would use a Proxy to intercept property access.

    • Answer: By defining a handler with a `get` trap in the Proxy constructor, you intercept property access and can return custom values or perform actions.

70. Explain how to use a Proxy to intercept property assignments.

    • Answer: Using a handler with a `set` trap, you can intercept property assignment attempts and perform validation or other actions before actually assigning the value.

71. How would you use the `Reflect` API to get the value of a property?

    • Answer: `Reflect.get(object, propertyKey)`

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