Please share JavaScript interview questions and answers
Sure! Below are some common JavaScript interview questions, along with their answers. These questions range from basic to advanced levels.
### Basic Questions
1. **What is JavaScript?**
- **Answer:** JavaScript is a high-level, interpreted programming language primarily used for enhancing web pages. It allows for dynamic content, interactive features, and is an essential part of web development alongside HTML and CSS. JavaScript can also be used on the server side with environments like Node.js.
2. **What are variables in JavaScript?**
- **Answer:** Variables in JavaScript are used to store data values. They can be declared using `var`, `let`, or `const`.
- `var`: Function-scoped or globally-scoped.
- `let`: Block-scoped, not hoisted.
- `const`: Block-scoped and cannot be reassigned after its initial assignment.
3. **What are data types in JavaScript?**
- **Answer:** JavaScript has several data types:
- **Primitive Types:** Undefined, Null, Boolean, Number, BigInt, String, Symbol.
- **Non-Primitive Type:** Object.
4. **What is a closure in JavaScript?**
- **Answer:** A closure is a function that has access to its own scope, the scope of the outer function, and the global scope. Closures are created every time a function is created. They are useful for data encapsulation and creating private variables.
5. **Explain the difference between `==` and `===` in JavaScript.**
- **Answer:** `==` is the equality operator that checks for equality of values but ignores the type, performing type coercion if necessary. `===` is the strict equality operator that checks for equality of both value and type and does not perform type coercion.
### Intermediate Questions
6. **What is an IIFE (Immediately Invoked Function Expression)?**
- **Answer:** An IIFE is a function that is declared and executed immediately. It helps in creating a new scope and avoiding global scope pollution.
```javascript
(function() {
// code here
})();
```
7. **What is event delegation?**
- **Answer:** Event delegation is a technique in which a single event listener is added to a parent element instead of multiple listeners on individual child elements. This approach allows us to manage events efficiently and is beneficial for dynamically added elements.
8. **What are Promises in JavaScript?**
- **Answer:** A Promise is a JavaScript feature used for asynchronous programming. It represents a value that may be available now, or in the future, or never. A Promise can be in one of three states: pending, fulfilled, or rejected. You can use `.then()` to handle fulfilled promises and `.catch()` to handle rejected ones.
9. **What is the `this` keyword in JavaScript?**
- **Answer:** The `this` keyword refers to the object that is executing the current function. Its value can change based on how a function is called:
- In a method, `this` refers to the object the method is called on.
- In a regular function, `this` refers to the global object (window in browsers).
- In an arrow function, `this` is lexically bound to the enclosing scope.
### Advanced Questions
10. **What is the event loop in JavaScript?**
- **Answer:** The event loop is a mechanism that allows JavaScript to perform non-blocking operations despite being single-threaded. It continuously checks the call stack and the message queue, executing functions in the stack until it's empty, then processing messages in the queue.
11. **Explain prototypal inheritance.**
- **Answer:** Prototypal inheritance is a feature in JavaScript that allows one object to inherit properties and methods from another object. This is achieved through the prototype chain, where an object’s prototype can be set to another object, and properties or methods can be looked up in the prototype chain if they are not found in the object itself.
12. **What are generators in JavaScript?**
- **Answer:** Generators are a special type of function that can be paused and resumed. They are defined using the `function*` syntax and use `yield` to pause execution. This allows for the creation of iterator objects that can produce a sequence of values over time.
13. **What is the difference between `null` and `undefined`?**
- **Answer:** `undefined` means a variable has been declared but has not yet been assigned a value. `null` is an assignment value and represents 'no value' or 'empty value'. Both are falsy in boolean contexts.
14. **What is the purpose of the `bind()` method?**
- **Answer:** The `bind()` method creates a new function that, when called, has its `this` keyword set to the provided value. It can also take additional arguments that will be pre-filled when the new function is called.
15. **How does the spread operator work in JavaScript?**
- **Answer:** The spread operator (`...`) allows an iterable (like an array or a string) to be expanded in places where zero or more arguments or elements are expected. It can be used for function calls, array literals, and object literals.
```javascript
const arr1 = [1, 2, 3];
const arr2 = [4, 5, ...arr1]; // arr2 is [4, 5, 1, 2, 3]
```
These questions and answers should help give you a solid foundation for JavaScript interviews. Feel free to ask for clarification or more specific areas of focus!