JavaScript Interview Questions and Answers for experienced

1. What is the difference between `==` and `===` in JavaScript?

   • Answer: `==` performs loose equality comparison, while `===` performs strict equality comparison. Loose equality performs type coercion if necessary before comparing values, while strict equality only returns true if both values are of the same type and have the same value.

2. Explain 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. This doesn't apply to assignments; only the declaration is hoisted. Therefore, a variable declared with `var` will be hoisted with an initial value of `undefined`, while `let` and `const` declarations are hoisted but remain in a "temporal dead zone" until their declaration is reached, resulting in a `ReferenceError` if accessed prematurely.

3. What are closures in JavaScript?

   • Answer: A closure is a function that has access to the variables in its surrounding scope, even after that scope has finished executing. This allows functions to "remember" their environment, enabling features like state preservation and encapsulation.

4. Explain the `this` keyword in JavaScript.

   • Answer: The `this` keyword refers to the object that is executing the current function. Its value depends on how the function is called (e.g., method invocation, function invocation, `call`, `apply`, `bind`).

5. What are prototypes in JavaScript?

   • Answer: Prototypes are the mechanism by which JavaScript objects inherit properties and methods from other objects. Every object has a prototype (except `null`), which is another object. When you try to access a property on an object, JavaScript will first check the object itself, and if it doesn't find the property, it will look in the object's prototype, then the prototype's prototype, and so on, until it reaches the end of the prototype chain (which is `null`).

6. Explain the difference between `let`, `const`, and `var`.

   • Answer: `var` has function scope (or global if not inside a function). `let` and `const` have block scope (within `{}`). `const` declares a constant value, meaning its value cannot be reassigned after initialization. `let` allows reassignment.

7. What are promises in JavaScript?

   • Answer: Promises are a way to handle asynchronous operations in JavaScript. A promise represents the eventual result of an asynchronous operation, and can be in one of three states: pending, fulfilled (resolved), or rejected. They provide a cleaner alternative to callbacks for handling asynchronous code.

8. What is async/await?

   • Answer: `async/await` is syntactic sugar built on top of promises that makes asynchronous code look and behave a bit more like synchronous code. `async` declares an asynchronous function, and `await` pauses execution of the `async` function until a promise is resolved or rejected.

9. Explain event bubbling and event capturing.

   • Answer: Event bubbling is the process where an event triggered on an element will also trigger on its parent elements. Event capturing is the reverse; the event is first handled by the outermost element and then propagates down to the target element. You can control which phase you listen for using `addEventListener`'s third parameter (useCapture).

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

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

11. What are some common JavaScript design patterns?

    • Answer: Common JavaScript design patterns include Module, Singleton, Factory, Observer, Decorator, Strategy, and many others. Each pattern solves a specific problem and promotes code reusability and maintainability.

12. How do you handle errors in JavaScript?

    • Answer: JavaScript uses `try...catch...finally` blocks to handle errors. The `try` block contains the code that might throw an error. The `catch` block handles the error if one is thrown. The `finally` block (optional) executes regardless of whether an error occurred.

13. 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.

14. How do you make an AJAX request in JavaScript?

    • Answer: Traditionally, `XMLHttpRequest` was used. Now, the `fetch` API provides a more modern and cleaner way to make AJAX requests.

15. What is the DOM?

    • Answer: The DOM (Document Object Model) is a programming interface for HTML and XML documents. It represents the page so that programs can change the document structure, style, and content.

16. Explain how to use `map`, `filter`, and `reduce` array methods.

    • Answer: `map` creates a new array by applying a function to each element of an existing array. `filter` creates a new array containing only elements that pass a certain condition. `reduce` applies a function cumulatively to the array elements to reduce them to a single value.

17. What are some ways to improve JavaScript performance?

    • Answer: Minimizing DOM manipulations, using efficient algorithms and data structures, minimizing the number of HTTP requests, using caching, code optimization (minification, tree-shaking), and using appropriate data structures are some ways.

18. What is a JavaScript framework, and name some popular ones?

    • Answer: A JavaScript framework is a collection of pre-written JavaScript code that provides structure and functionality to web applications. Popular examples include React, Angular, Vue.js, Node.js (server-side).

19. Explain the concept of event delegation.

    • Answer: Event delegation is a technique where you attach an event listener to a parent element, and then use event bubbling to handle events triggered on its children. This is more efficient than attaching listeners to each individual child element, especially when dealing with a large number of children.

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