ES6 Interview Questions and Answers for experienced

1. What are the key features introduced in ES6 (ECMAScript 2015)?

   • Answer: ES6 introduced many significant features, including arrow functions, classes, modules, `let` and `const` declarations, template literals, destructuring, promises, iterators and generators, Map and Set objects, and more. These features improved code readability, organization, and efficiency.

2. Explain arrow functions and their advantages over traditional functions.

   • Answer: Arrow functions provide a concise syntax for writing functions. They lexically bind `this`, simplifying handling of `this` within callbacks and object methods. They also implicitly return single-expression results, making them more compact. However, they lack their own `this`, `arguments`, and `super` keywords, which can be limitations in certain contexts.

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

   • Answer: `var` has function scope (or global if declared outside a function), while `let` and `const` have block scope. `const` declares a constant, meaning its value cannot be reassigned after initialization. `let` allows reassignment. `var` is generally discouraged in modern JavaScript due to its potential for hoisting-related issues and its broader scope.

4. Explain template literals and their benefits.

   • Answer: Template literals (backticks ``) allow embedded expressions within strings using `${expression}`. This improves string formatting and makes creating dynamic strings much cleaner and more readable than traditional string concatenation.

5. How does destructuring work in ES6? Provide examples.

   • Answer: Destructuring allows extracting values from objects and arrays into distinct variables. For example: `const { name, age } = person;` extracts `name` and `age` from the `person` object. `const [first, second] = array;` extracts the first two elements of an array. This simplifies variable assignment and improves code clarity.

6. What are Promises and how do they handle asynchronous operations?

   • Answer: Promises represent the eventual result of an asynchronous operation. They have three states: pending, fulfilled (success), and rejected (failure). They allow chaining asynchronous operations using `.then()` for success and `.catch()` for failure handling, making asynchronous code more manageable and readable than callbacks.

7. Explain the difference between `async` and `await`.

   • Answer: `async` declares an asynchronous function that implicitly returns a Promise. `await` pauses execution inside an `async` function until a Promise resolves, making asynchronous code look and behave a bit more like synchronous code, improving readability.

8. What are iterators and generators in ES6?

   • Answer: Iterators provide a standardized way to traverse data structures. Generators are special functions that can pause execution and resume later, yielding values one at a time. They are useful for creating custom iterators and handling large datasets efficiently.

9. Describe the Map and Set objects.

   • Answer: `Map` is a collection of key-value pairs where keys can be of any data type. `Set` is a collection of unique values. Both offer efficient methods for adding, removing, and checking for the existence of elements, providing alternatives to arrays and objects in specific scenarios.

10. Explain how modules work in ES6.

    • Answer: ES6 modules provide a mechanism for organizing code into reusable units. Modules can export functions, classes, and variables, and other modules can import them using `import` and `export` statements. This promotes code reusability, modularity, and maintainability.

11. What are classes in ES6 and how do they differ from prototypal inheritance?

    • Answer: ES6 classes provide syntactic sugar over JavaScript's prototypal inheritance. They offer a more familiar class-based syntax for defining objects and inheritance, but ultimately, they still rely on prototypes under the hood. Classes make object-oriented programming concepts easier to express and understand in JavaScript.

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

    • Answer: The `try...catch` block allows handling errors gracefully. Code that might throw an error is placed within the `try` block. If an error occurs, it's caught by the `catch` block, allowing for error handling and preventing the program from crashing.

13. Explain the spread syntax (...) and rest parameters (...).

    • Answer: The spread syntax expands iterable objects into individual elements. Rest parameters gather multiple function arguments into a single array. Both are powerful tools for working with arrays and function arguments in a more concise way.

14. What are Symbols in ES6?

    • Answer: Symbols are unique and immutable data types, often used to create unique object properties that are unlikely to collide with other properties. They help avoid naming conflicts and provide a way to create private properties in a sense.

15. How to use `Object.assign()`?

    • Answer: `Object.assign()` copies the values of all enumerable own properties from one or more source objects to a target object. It's useful for creating copies of objects or merging objects together.

16. Explain `Object.keys()`, `Object.values()`, and `Object.entries()`.

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

17. What are Proxies in ES6?

    • Answer: Proxies allow intercepting and customizing operations performed on an object. You can define custom behavior for getting, setting, and deleting properties, among other actions. They are powerful for creating advanced object behaviors and data manipulation.

18. What are Reflect API methods?

    • Answer: The Reflect API provides methods that perform the same operations as the corresponding methods on `Object`, but in a more functional style. They're often used in conjunction with Proxies.

19. Explain the difference between `forEach`, `map`, `filter`, and `reduce`.

    • Answer: `forEach` iterates over an array and executes a provided function for each element. `map` creates a new array by applying a function to each element. `filter` creates a new array containing only elements that pass a provided test function. `reduce` applies a function cumulatively to each element to reduce the array to a single value.

20. What is a WeakMap and when would you use it?

    • Answer: A `WeakMap` is a collection of key-value pairs where keys must be objects and the keys are weakly referenced. If there are no other references to the key object, the key-value pair will be garbage collected. This is useful for managing caches or associating data with DOM elements without preventing garbage collection.

21. What is a WeakSet and when would you use it?

    • Answer: A `WeakSet` is similar to a `Set`, but its elements must be objects and are weakly referenced. If there are no other references to an object in the `WeakSet`, it will be garbage collected. This is useful when you need to track objects without preventing garbage collection.

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

    • Answer: The value of `this` depends on how the function is called. In arrow functions, `this` is lexically bound, while in regular functions, it's determined by how the function is invoked (e.g., as a method of an object, in strict mode, etc.).

23. How can you create a private class member in ES6? (Approaches and limitations).

    • Answer: While ES6 doesn't have true private members like some other languages, using a WeakMap can effectively create private state that's not directly accessible from outside the class. It's important to understand that this is a convention, not a language feature, and there are workarounds to access it, so "privacy" is relative.

24. What are tagged template literals? Give an example.

    • Answer: Tagged template literals allow you to pass template literal strings to a function for processing before they are evaluated. The function receives the string parts as an array and the expressions as separate arguments. This is useful for creating custom string formatting, sanitizing, or other transformations.

25. How does the `finally` block work in a `try...catch...finally` statement?

    • Answer: The `finally` block is always executed regardless of whether an error occurs or not. It's often used for cleanup operations, like closing files or releasing resources.

26. Explain the difference between a regular function and a generator function.

    • Answer: A regular function executes and returns a single value. A generator function uses `function*` and `yield` to pause execution and return multiple values over time. This is useful for iterating over sequences and creating custom iterators.

27. How can you create a class that extends another class in ES6?

    • Answer: Use the `extends` keyword. For example: `class SubClass extends SuperClass { ... }`

28. Explain static methods in ES6 classes.

    • Answer: Static methods belong to the class itself, not to instances of the class. They are called using the class name, not an object instance. They are useful for utility functions related to the class.

29. How do you implement inheritance in ES6 using classes?

    • Answer: Use the `extends` keyword to create a subclass that inherits from a superclass. The subclass can override methods from the superclass and add its own methods and properties.

30. What is method chaining and how can you achieve it in ES6?

    • Answer: Method chaining is calling multiple methods on an object sequentially. This is achieved by having each method return the object itself (`this`).

31. Explain the concept of closures in ES6.

    • Answer: A closure is a function that has access to variables from its surrounding scope, even after that scope has finished executing. This is a fundamental concept in JavaScript.

32. How to create an immutable object in ES6?

    • Answer: There isn't a built-in way to create completely immutable objects, but techniques like using `Object.freeze()` can prevent modifications, and creating new objects instead of modifying existing ones helps maintain immutability.

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

    • Answer: Use `let` and `const` instead of `var`. Favor arrow functions for concise syntax, but be mindful of their limitations. Use modules for better code organization. Employ destructuring for cleaner code. Choose appropriate data structures (Map, Set, etc.) for specific tasks.

34. How can you handle asynchronous operations using Promises and async/await together?

    • Answer: Use `async` to declare an asynchronous function and `await` to pause execution until a Promise resolves. This makes asynchronous code much more readable.

35. Explain the difference between `null` and `undefined` in ES6.

    • Answer: `null` is a deliberate assignment indicating the absence of a value. `undefined` means a variable has been declared but not assigned a value.

36. How can you use the `for...of` loop?

    • Answer: The `for...of` loop iterates over iterable objects like arrays, strings, Maps, and Sets.

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

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

38. How can you debug ES6 code effectively?

    • Answer: Use your browser's developer tools (debugger statements, breakpoints, console logging) or a dedicated JavaScript debugger.

39. Explain how to use the `import` and `export` keywords in ES modules.

    • Answer: `export` makes variables, functions, or classes available to other modules. `import` brings those exported elements into the current module.

40. What are some common pitfalls to avoid when using ES6?

    • Answer: Misunderstanding `this` in regular functions, incorrect use of `const` (trying to reassign), improper error handling, and neglecting module organization.

41. How can you efficiently handle large arrays in ES6?

    • Answer: Use appropriate array methods like `map`, `filter`, `reduce`, and generators to process data in chunks or use libraries optimized for large data sets.

42. How do you deal with asynchronous errors in ES6 using Promises?

    • Answer: Use the `.catch()` method of a Promise to handle any errors that occur during the asynchronous operation.

43. Explain the use of `Symbol.iterator` for creating iterable objects.

    • Answer: `Symbol.iterator` is a well-known symbol used to define an iterator method for custom objects, making them iterable using `for...of` loops.

44. How can you improve code readability and maintainability when working with ES6?

    • Answer: Use meaningful variable and function names, follow consistent code style, break down complex tasks into smaller functions, use comments effectively, and leverage ES6 features to write concise and expressive code.

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