JavaScript Interview Questions and Answers for 5 years experience

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

   • Answer: == performs loose equality comparison, converting types before comparison. === performs strict equality comparison, requiring both value and type to be the same. For example, 1 == "1" is true, but 1 === "1" is false.

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 means you can use a variable or function before its declaration, although it's generally considered best practice to declare them before use for readability and to avoid unexpected behavior.

3. What are closures in JavaScript? Give an example.

   • 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. Example: ```javascript function outerFunction() { let outerVar = "Hello"; function innerFunction() { console.log(outerVar); // innerFunction has access to outerVar } return innerFunction; } let myClosure = outerFunction(); myClosure(); // Outputs "Hello" ```

4. Explain the concept of prototypal inheritance in JavaScript.

   • Answer: JavaScript uses prototypal inheritance, where objects inherit properties and methods from a prototype object. Every object has a prototype, and when you try to access a property on an object, JavaScript will first look for it on the object itself. If not found, it searches the prototype chain until it finds the property or reaches the end of the chain (Object.prototype).

5. What are the different ways to create objects in JavaScript?

   • Answer: Objects can be created using object literals {}, constructor functions, the Object.create() method, and classes (ES6 and later).

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

   • Answer: var is function-scoped (or globally scoped if not inside a function). let and const are block-scoped. const declares a constant whose value cannot be reassigned after initialization. let allows reassignment.

7. Explain the concept of asynchronous programming in JavaScript.

   • Answer: Asynchronous programming allows JavaScript to handle multiple tasks concurrently without blocking the main thread. This is crucial for I/O operations (network requests, file system access) to prevent the UI from freezing. Techniques like callbacks, promises, and async/await are used to manage asynchronous operations.

8. What are promises in JavaScript and how do you use them?

   • Answer: Promises represent the eventual result of an asynchronous operation. They have three states: pending, fulfilled (success), and rejected (failure). They are used with `.then()` for handling success, and `.catch()` for handling errors.

9. Explain `async/await` in JavaScript.

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

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