ES6 Interview Questions and Answers for 5 years experience

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

   • Answer: ES6 introduced many significant features, including arrow functions, `let` and `const` declarations, classes, modules, template literals, destructuring, promises, iterators and generators, Map and Set objects, and more. These features significantly improved JavaScript's conciseness, readability, and capabilities.

2. Explain arrow functions and their benefits.

   • 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. They're also concise and often lead to more readable code, especially in functional programming paradigms.

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

   • Answer: `var` has function scope, while `let` and `const` have block scope. `const` declares a constant value, meaning it cannot be reassigned after initialization. `let` allows reassignment but is block-scoped, preventing hoisting issues.

4. Explain how to use destructuring in ES6. Provide examples.

   • Answer: Destructuring allows assigning values from objects or arrays to distinct variables in a concise way. Example: `const {name, age} = {name: 'John', age: 30};` or `const [first, second] = [1, 2];` It simplifies code and improves readability when working with complex data structures.

5. What are template literals? Give examples.

   • Answer: Template literals use backticks (`) to define strings. They allow embedded expressions using `${expression}` and multi-line strings without concatenation. Example: `const name = "John"; const greeting = `Hello, ${name}!`;`

6. Explain the concept of Promises in JavaScript.

   • Answer: Promises represent the eventual result of an asynchronous operation. They handle asynchronous code more cleanly than callbacks by providing `.then()` for success and `.catch()` for error handling. They improve code readability and manageability for asynchronous tasks.

7. How do you use `async/await` with Promises?

   • Answer: `async/await` makes asynchronous code look and behave a bit more like synchronous code, making it easier to read and reason about. `async` declares an asynchronous function, and `await` pauses execution until a Promise resolves.

8. Describe the differences between `Map` and `Object` in ES6.

   • Answer: `Map` objects allow any data type as keys (including objects), while `Object` keys are always strings. `Map` provides methods like `set()`, `get()`, `has()`, and `delete()`, giving more explicit control over key-value pairs. `Map` also maintains insertion order, unlike `Object`.

9. Explain the use of `Set` in ES6.

   • Answer: `Set` objects store unique values of any type. They offer methods to add, delete, and check for the existence of elements, ensuring that only one instance of each value is present. They are useful for tasks like removing duplicates from an array.

10. What are iterators and generators in ES6?

    • Answer: Iterators provide a standard way to traverse data structures. Generators are special functions that can be paused and resumed, producing values one at a time. They're useful for creating custom iterators and working with large datasets efficiently.

11. How do ES6 modules work? Explain `import` and `export`.

    • Answer: ES6 modules provide a standard way to import and export code between files. `export` makes variables, functions, or classes available to other modules, while `import` brings in those exported elements. This promotes code reusability and organization.

12. What are classes in ES6? How are they different from constructor functions?

    • Answer: ES6 classes provide a syntactic sugar over prototype-based inheritance. They offer a cleaner and more readable way to define objects and their inheritance compared to constructor functions. They provide a `constructor` method for object initialization and use the `extends` keyword for inheritance.

13. Explain the use of `Symbol` in ES6.

    • Answer: Symbols are unique and immutable data types, often used to create unique object properties that are less likely to collide with other properties, ensuring better privacy and reducing naming conflicts.

14. What is the spread syntax (...) in ES6? Give examples.

    • Answer: The spread syntax expands iterables (arrays, objects, strings, etc.) into individual elements. It's used for array copying, combining arrays, and passing multiple arguments to functions. Example: `const newArray = [...oldArray, newItem];`

15. What is the rest parameter (...) in ES6? Give examples.

    • Answer: The rest parameter gathers multiple arguments into an array. This simplifies function definitions when you don't know how many arguments will be passed. Example: `function sum(...numbers) { return numbers.reduce((a, b) => a + b, 0); }`

16. How to handle errors using `try...catch` in ES6?

    • Answer: The `try...catch` block handles exceptions during code execution. The `try` block contains the code that might throw an error, and the `catch` block handles the error if one occurs, preventing the program from crashing.

17. Explain `Proxy` objects in ES6.

    • Answer: `Proxy` objects allow intercepting and customizing operations on another object (the target). This is useful for tasks like data validation, logging, or creating virtual properties.

18. Explain `Reflect` objects in ES6.

    • Answer: `Reflect` provides methods that mirror JavaScript's internal behavior, making it easier to work with object properties and other internal operations. It often works in conjunction with `Proxy` objects.

19. What are tagged template literals?

    • Answer: Tagged template literals allow you to call a function with the string parts and expressions of a template literal as arguments. This enables custom string processing and manipulation.

20. Explain `WeakMap` and `WeakSet` in ES6.

    • Answer: `WeakMap` and `WeakSet` are collections that don't prevent garbage collection of their keys (for `WeakMap`) or values (for `WeakSet`). They are particularly useful when you don't want to keep objects in memory longer than necessary.

21. What are the benefits of using modules in larger JavaScript projects?

    • Answer: Modules promote code organization, reusability, and maintainability. They prevent naming conflicts and make it easier to manage dependencies in large projects.

22. How can you handle asynchronous operations efficiently in a large application?

    • Answer: Use Promises and `async/await` for cleaner asynchronous code. Consider using libraries like Lodash or Ramda for functional programming techniques to handle asynchronous operations in a more manageable way.

23. Explain the concept of closures in JavaScript, and how they relate to ES6 features.

    • Answer: Closures are functions that have access to variables from their surrounding scope, even after that scope has finished executing. This is important in ES6 with arrow functions and modules, as they often rely on lexical scoping.

24. How do you debug ES6 code effectively?

    • Answer: Use your browser's developer tools (e.g., Chrome DevTools) for setting breakpoints, stepping through code, inspecting variables, and using the console for logging. Linters and code formatters also help prevent errors and improve code readability.

25. What are some common pitfalls to avoid when working with ES6 features?

    • Answer: Be mindful of lexical `this` in arrow functions. Avoid unnecessary reassignments with `const`. Understand the differences between `Map` and `Object` and choose the appropriate one for your use case. Handle asynchronous operations carefully to avoid callback hell.

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