Interview Questions for Web Developer - JavaScript

Here's a set of interview questions for a Web Developer focusing on JavaScript, along with their answers:

Basic JavaScript:

1. What is JavaScript? How does it differ from Java?

Answer: 

JavaScript is a scripting language used to create and control dynamic website content. Unlike Java, it is a scripting language interpreted by browsers, whereas Java is a programming language that needs a runtime environment.

2. Explain the difference between `null`, `undefined`, and `undeclared` variables.

Answer:

   - `null` represents an intentional absence of any object value.

   - `undefined` means a variable has been declared but not assigned a value.

   - `undeclared` variables are ones that have been used without being declared using `var`, `let`, or `const`.

3. Describe the difference between `==` and `===` operators.

Answer: 

   - `==` is an equality operator that checks for equality after performing type coercion.

   - `===` is a strict equality operator that checks for both value and type equality without coercion.

4. What is the purpose of using `let`, `const`, and `var` in variable declaration? How do they differ?

Answer:

   - `let` and `const` are block-scoped variables introduced in ES6. `let` allows variable reassignment, while `const` creates constants that can't be reassigned.

   - `var` is function-scoped and hoisted, allowing redeclaration and updating its value.

5. Explain hoisting in JavaScript.

Answer: 

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase, before execution.

6. What is the Event Loop in JavaScript?

Answer: 

The Event Loop manages the execution of multiple code fragments by putting events and callbacks in a queue and executing them one by one in the main thread.

7. What is a closure? Can you provide an example of its practical use?

 Answer:

A closure is an inner function that has access to the outer (enclosing) function's variables. It has three scope chains: its own scope, the outer function's variables, and global variables. An example:

   function outerFunction() {

     let outerVar = 'I am from outerFunction';

     function innerFunction() {

       console.log(outerVar);

     }

     return innerFunction;

   }

   let newFunc = outerFunction();

   newFunc(); // Output: "I am from outerFunction"

Advanced JavaScript:

8. What is the prototype chain in JavaScript?

Answer:

The prototype chain is a series of objects linked through their prototype objects, where an object inherits properties and methods from its prototype object.

9. Explain how `async/await` works and how it is different from using promises.

Answer:

   - `async/await` is built on top of promises and provides a cleaner syntax for handling asynchronous operations in JavaScript.

   - It allows writing asynchronous code that looks synchronous, making it easier to read and maintain.

10. What is the purpose of IIFE (Immediately Invoked Function Expression)?

Answer:

IIFE is a JavaScript function that runs as soon as it is defined. It is used to create a private scope and avoid polluting the global namespace.

11. How does JavaScript handle asynchronous code execution? Discuss callbacks, Promises, and async/await.

Answer: 

JavaScript handles asynchronous code execution using:

    - Callbacks: Functions passed as arguments to other functions and executed when an operation is complete.

    - Promises: Objects representing the eventual completion or failure of an asynchronous operation.

    - async/await: Built on top of promises, providing a more readable and synchronous-like way to write asynchronous code.

12. Explain the concept of currying in JavaScript.

Answer: 

Currying is a technique of converting a function with multiple arguments into a sequence of nested functions. Each function takes a single argument until all arguments are fulfilled.

13. How does `this` keyword work in JavaScript? Explain with examples.

Answer: 

The `this` keyword refers to the object it belongs to. Its value is determined by how a function is called, and it can change based on the context of its execution. Example:

    let obj = {

      prop: 'I am obj',

      showProp: function() {

        console.log(this.prop);

      }

    }

    obj.showProp(); // Output: "I am obj"

14. Describe the differences between ES6 class and prototype-based inheritance.

Answer: 

    - ES6 classes are syntactical sugar over JavaScript's existing prototype-based inheritance, providing a cleaner syntax to create classes and deal with inheritance.

    - Prototype-based inheritance involves creating objects and then extending them by adding methods and properties to their prototype.

These questions and answers cover a range of JavaScript concepts, from basic syntax to advanced topics. Adjust the difficulty level according to the candidate's experience and the specific requirements of the role.