ES6 Interview Questions and Answers

1. What is ES6 (ECMAScript 2015)?

   • Answer: ES6, also known as ECMAScript 2015, is a major update to the JavaScript language that introduced many new features to improve code readability, maintainability, and efficiency. These features include arrow functions, classes, modules, promises, and more.

2. Explain arrow functions. What are their advantages and disadvantages?

   • Answer: Arrow functions provide a shorter syntax for writing functions. They lexically bind `this`, eliminating the need for `bind`, `call`, or `apply` in many cases. Advantages: Concise syntax, lexical `this` binding. Disadvantages: Don't have their own `this` binding, cannot be used as constructors, lack `arguments` object.

3. What are let and const? How do they differ from var?

   • Answer: `let` and `const` are block-scoped variables, meaning their scope is limited to the block of code (defined by curly braces `{}`) they are declared in. `var` is function-scoped or globally scoped. `const` declares a constant whose value cannot be reassigned after initialization. `let` allows reassignment.

4. Explain the concept of destructuring.

   • Answer: Destructuring allows you to unpack values from arrays or objects into distinct variables. This makes code cleaner and more readable, especially when working with objects containing multiple properties.

5. How do you use template literals?

   • Answer: Template literals use backticks (`) instead of single or double quotes. They allow embedding expressions directly within strings using `${expression}`. This simplifies string concatenation and improves readability.

6. What are classes in ES6?

   • Answer: ES6 classes provide a syntactic sugar over JavaScript's prototype-based inheritance. They offer a cleaner and more familiar way to define objects and their methods, resembling class-based object-oriented programming.

7. Explain the difference between `class` and `prototype` inheritance.

   • Answer: `class` syntax is a more readable and maintainable way to achieve prototype-based inheritance. Under the hood, classes still use prototypes; the `class` keyword simply provides a cleaner syntax.

8. What are modules in ES6? How do they work?

   • Answer: ES6 modules provide a way to organize code into reusable units. They use `export` to make parts of a module available to other modules and `import` to use those exported parts. This promotes modularity and code reusability.

9. Explain promises in ES6.

   • Answer: Promises represent the eventual result of an asynchronous operation. They handle asynchronous operations more elegantly than callbacks, avoiding "callback hell" by using `.then()` for success and `.catch()` for error handling.

10. What is `async/await`? How does it work with promises?

    • 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 resolves (or rejects).

11. What are iterators and generators?

    • Answer: Iterators provide a standardized way to traverse data structures. Generators are a special type of function that can be paused and resumed, making them useful for creating iterators and handling asynchronous operations more efficiently.

12. Explain the `Map` object.

    • Answer: `Map` is a collection of key-value pairs where keys can be of any data type (unlike objects where keys are strings).

13. Explain the `Set` object.

    • Answer: `Set` is a collection of unique values. It's useful for removing duplicates from an array or checking if a value exists.

14. What are Symbols in ES6?

    • Answer: Symbols are unique and immutable data types. They are often used as unique keys for object properties, preventing naming collisions.

15. Explain the spread syntax (...).

    • Answer: The spread syntax allows you to spread the elements of an iterable (like an array) into individual elements. It's used for copying arrays, merging arrays, and passing arguments to functions.

16. Explain the rest parameter syntax (...).

    • Answer: The rest parameter syntax allows a function to accept an indefinite number of arguments as an array. This is useful for creating functions that can handle a variable number of inputs.

17. What is the difference between `Object.assign()` and the spread operator for copying objects?

    • Answer: Both copy objects, but `Object.assign()` performs a shallow copy, while the spread operator can also perform a shallow copy but is generally more concise and readable.

18. How do you handle errors using `try...catch`?

    • Answer: `try...catch` blocks handle exceptions that might occur during code execution. Code that might throw an error is placed in the `try` block, and the `catch` block handles any errors that occur.

19. What are Proxies?

    • Answer: Proxies allow you to intercept and customize operations performed on an object. This is powerful for features like logging, validation, or creating custom getters and setters.

20. What are Reflect API methods?

    • Answer: The Reflect API provides methods for performing operations on objects, similar to the methods available on `Object`, but with a slightly different approach, often aiming for consistency and better error handling.

21. Explain `for...of` and `for...in` loops. What are their differences?

    • Answer: `for...of` iterates over iterable objects (arrays, strings, maps, sets), providing the value of each element. `for...in` iterates over the enumerable properties of an object, providing the key of each property.

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

    • Answer: `==` performs loose equality comparison, performing type coercion if necessary. `===` performs strict equality comparison, requiring both value and type to be the same.

23. How can you create a private class member in ES6 (or a close approximation)?

    • Answer: ES6 doesn't have true private class members. However, using a WeakMap, you can create a mechanism that achieves a similar effect, preventing direct access from outside the class.

24. Explain the concept of hoisting in JavaScript, and how it relates to `let` and `const`.

    • Answer: Hoisting is the process where JavaScript moves declarations to the top of their scope before execution. `var` declarations are hoisted, but `let` and `const` are not hoisted in the same way; they are subject to the Temporal Dead Zone (TDZ).

25. What is the Temporal Dead Zone (TDZ)?

    • Answer: The TDZ is the region of code between the start of a block scope and the declaration of a `let` or `const` variable. Accessing a variable within the TDZ results in a `ReferenceError`.

26. How do you import and export default members from modules?

    • Answer: You use `export default` to export a single default member from a module and `import` (without curly braces) to import it.

27. How do you import and export named members from modules?

    • Answer: You use `export` to name members for export and `import` (with curly braces) to import named members.

28. Explain the concept of inheritance in ES6 classes.

    • Answer: ES6 classes support inheritance using the `extends` keyword. A subclass inherits properties and methods from its superclass and can override or extend them.

29. What is the `super` keyword used for in ES6 classes?

    • Answer: The `super` keyword is used to call the constructor or methods of the superclass from within a subclass.

30. How can you create a static method in an ES6 class?

    • Answer: Use the `static` keyword before the method declaration to create a static method that belongs to the class itself, not instances of the class.

31. What is a getter and setter in ES6 classes?

    • Answer: Getters and setters provide controlled access to class properties. Getters retrieve the value, and setters modify the value, allowing for custom logic during access.

32. What are some common use cases for Promises?

    • Answer: Handling asynchronous operations like fetching data from an API, performing file I/O, or waiting for timers.

33. Explain the concept of chaining promises with `.then()`.

    • Answer: `.then()` returns a new promise, allowing you to chain multiple asynchronous operations together in a sequential manner.

34. How do you handle errors in a chain of promises?

    • Answer: Use `.catch()` to handle errors that might occur in any part of the promise chain.

35. What is Promise.all()?

    • Answer: `Promise.all()` takes an array of promises and resolves when all promises in the array have resolved, or rejects if any of them reject.

36. What is Promise.race()?

    • Answer: `Promise.race()` takes an array of promises and resolves or rejects as soon as the first promise in the array resolves or rejects.

37. Explain the use of `finally` in async/await.

    • Answer: The `finally` block in an `async/await` function always executes regardless of whether the `await`ed promise resolves or rejects.

38. How do you create a generator function?

    • Answer: Use the `function*` syntax to define a generator function.

39. How do you use `yield` in a generator function?

    • Answer: `yield` pauses the generator function's execution and returns a value. The generator can be resumed using `.next()`.

40. What are the benefits of using generators?

    • Answer: Generators allow you to write efficient code for iterating over large datasets or handling long-running processes without blocking the main thread.

41. How can you check if an object is iterable?

    • Answer: Check if the object has a `Symbol.iterator` property.

42. Explain the difference between a Map and a WeakMap.

    • Answer: `Map` stores key-value pairs, while `WeakMap` only allows objects as keys and these objects are garbage collected if there are no other references to them.

43. Explain the difference between a Set and a WeakSet.

    • Answer: `Set` stores unique values, while `WeakSet` only allows objects as values and these objects are garbage collected if there are no other references to them. `WeakSet` doesn't provide iteration.

44. How do you create a Symbol?

    • Answer: Use `Symbol()` to create a new symbol.

45. What are some common use cases for Symbols?

    • Answer: Creating unique property keys to avoid naming collisions, creating hidden properties.

46. Explain the concept of a Proxy handler.

    • Answer: A Proxy handler is an object that specifies the traps (methods) that intercept operations on the target object.

47. What are some common Proxy traps?

    • Answer: `get`, `set`, `has`, `deleteProperty`, etc. These traps allow intercepting and customizing property access, modifications, and existence checks.

48. What are the benefits of using Proxies?

    • Answer: Proxies provide powerful ways to intercept and customize operations on objects, useful for features like validation, logging, and creating custom behavior.

49. How can you use `Reflect.get()`?

    • Answer: `Reflect.get()` is used to get a property value from an object; it's generally more consistent and safer than directly accessing the property using dot notation.

50. How can you use `Reflect.set()`?

    • Answer: `Reflect.set()` is used to set a property value on an object. Similar to `Reflect.get()`, it offers improved consistency.

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

    • Answer: `Object.keys()` returns an array of an object's own enumerable property names (keys).

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

    • Answer: `Object.values()` returns an array of an object's own enumerable property values.

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

    • Answer: `Object.entries()` returns an array of an object's own enumerable property [key, value] pairs.

54. Explain the `Number.isNaN()` method.

    • Answer: `Number.isNaN()` is a more reliable way to check if a value is NaN (Not a Number) compared to `isNaN()`.

55. Explain the `Number.isFinite()` method.

    • Answer: `Number.isFinite()` checks if a value is a finite number.

56. Explain the `Number.isInteger()` method.

    • Answer: `Number.isInteger()` checks if a value is an integer.

57. What is the difference between `parseInt()` and `parseFloat()`?

    • Answer: `parseInt()` parses a string and returns an integer. `parseFloat()` parses a string and returns a floating-point number.

58. How can you use the `Array.from()` method?

    • Answer: `Array.from()` creates a new array from an array-like or iterable object.

59. How can you use the `Array.of()` method?

    • Answer: `Array.of()` creates a new array from a variable number of arguments.

60. Explain the `Array.find()` method.

    • Answer: `Array.find()` returns the first element in an array that satisfies a provided testing function.

61. Explain the `Array.findIndex()` method.

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

62. Explain the `Array.includes()` method.

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

63. Explain the `Array.fill()` method.

    • Answer: `Array.fill()` changes all elements in an array to a static value, from a start index (default 0) to an end index (default array length).

64. Explain the `Array.copyWithin()` method.

    • Answer: `Array.copyWithin()` copies part of an array to another location in the same array and returns it without modifying its length.

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