Interview on JS /React
Tags: webdev, beginners, javascript, interview
Published: 2024-01-27 04:18:19

**Interviewer:** Can you arrange this array in ascending order with the help of any built-in function?

**Candidate:**  Absolutely, I can use the `sort` function to achieve that. Here's an example:

```javascript
var arr = [5, 2, 8, 1, 3];
arr.sort(function(a, b) {
  return a - b;
});
console.log(arr); // Output: [1, 2, 3, 5, 8]
```

**Interviewer:**  Great. What are the differences between `let` and `const`?

**Candidate:**  The main differences between `let` and `const` are in their reassignment behavior. `let` can be reassigned, while `const` cannot. Here's an example:

```javascript
let x = 10;
x = 20; // Valid with let

const y = 30;
y = 40; // Error with const
```

**Interviewer:**  Can you explain the concept of closures in JavaScript?

**Candidate:**  Certainly! Closures happen when a function is defined inside another function, allowing the inner function to access the outer function's variables. Here's a simple example:

```javascript
function outer() {
  let outerVar = 'I am outer!';

  function inner() {
    console.log(outerVar);
  }

  return inner;
}

const closureFunc = outer();
closureFunc(); // Output: I am outer!
```

**Interviewer:**  What is the `this` keyword in JavaScript?

**Candidate:**  The `this` keyword refers to the object to which a function or method belongs. Its value is determined by how a function is called. Here's a simple explanation:

```javascript
const person = {
  name: 'John',
  greet: function() {
    console.log(`Hello, ${this.name}!`);
  },
};

person.greet(); // Output: Hello, John!
```

**Interviewer:**  What is a promise in JavaScript?

**Candidate:**  A promise is an object representing the completion or failure of an asynchronous operation. It's like a placeholder for a future value. Here's a basic example:

```javascript
const myPromise = new Promise((resolve, reject) => {
  // Asynchronous operation
  const success = true;

  if (success) {
    resolve('Operation completed successfully!');
  } else {
    reject('Operation failed!');
  }
});

myPromise.then(result => console.log(result)).catch(error => console.error(error));
```

**Interviewer:**  Can you explain the event loop in JavaScript?

**Candidate:**  Absolutely! The event loop manages the execution of code in a non-blocking way. It's like a traffic cop for your asynchronous tasks. Here's a simplified analogy:

Imagine you're at a restaurant (event loop). You place an order (asynchronous task), and while waiting, you're free to chat with friends (execute other code). When your order is ready (asynchronous task completed), it gets served without blocking your conversations.

**Interviewer:**  What is the difference between synchronous and asynchronous code?

**Candidate:**  In simple terms, synchronous code is like waiting in a queue. Each task waits for the previous one to finish. Asynchronous code is like placing an order at a restaurant. You don't wait idly; you continue with other things, and your order arrives when ready.

**Interviewer:**  Can you explain rest and spread operators?

**Candidate:**  Certainly! So, the spread operator (`...`) is used for combining elements from different arrays or objects. For example, if we have arrays A and B, and we want to combine all elements into a new array C, we can use the spread operator like this:

```javascript
const arrA = [1, 2, 3];
const arrB = ['A', 'B', 'C'];
const arrC = [...arrA, ...arrB];

console.log(arrC); // Output: [1, 2, 3, 'A', 'B', 'C']
```

Now, the rest operator is used for function parameters. It allows a function to accept an indefinite number of arguments and treats them as an array. Here's an example:

```javascript
function sum(...numbers) {
  return numbers.reduce((acc, num) => acc + num, 0);
}

console.log(sum(1, 2, 3, 4)); // Output: 10
```

**Interviewer:**  Great! Can you explain the difference between push and unshift methods in JavaScript?

**Candidate:**  Absolutely! The `push` method is used to add elements to the end of an array, while the `unshift` method is used to add elements to the beginning of an array. Let me illustrate with an example:

```javascript
const numbers = [1, 2, 3];

// Using push to add element 4 to the end
numbers.push(4);
console.log(numbers); // Output: [1, 2, 3, 4]

// Using unshift to add element 0 to the beginning
numbers.unshift(0);
console.log(numbers); // Output: [0, 1, 2, 3, 4]
```

**Interviewer:**  Nice! Now, could you explain the concept of private routing in the context of web applications?

**Candidate:**  Certainly! Private routing is a mechanism in web applications where certain routes or pages are protected and only accessible if the user is authenticated or has the necessary permissions. For instance, in a banking application, the login page is a private route, as it requires valid user credentials. If a user tries to access a private route without authentication, they might be redirected to the login page.

**Interviewer:**  Good explanation! Moving on, can you describe the purpose of the `map` and `filter` functions in JavaScript?

**Candidate:**  Absolutely! The `map` function is used to transform each element of an array according to a provided function. For example:

```javascript
const numbers = [1, 2, 3];
const doubled = numbers.map(num => num * 2);
console.log(doubled); // Output: [2, 4, 6]
```

On the other hand, the `filter` function is used to create a new array with elements that pass a certain condition. Here's an example:

```javascript
const numbers = [1, 2, 3, 4];
const greaterThanTwo = numbers.filter(num => num > 2);
console.log(greaterThanTwo); // Output: [3, 4]
```

**Interviewer:**  Great examples! Finally, can you provide a brief explanation of the JSON format and its use?

**Candidate:**  Absolutely! JSON, or JavaScript Object Notation, is a lightweight data-interchange format. It uses a key-value pair structure to represent data, similar to JavaScript objects. It is commonly used for data exchange between a server and a web application. Here's a simple example:

```json
{
  "name": "John Doe",
  "age": 25,
  "city": "Example City"
}
```

This JSON represents an object with name, age, and city properties. It's widely used in web development for APIs, configuration files, and more.

**Interviewer:** : Can you arrange this array in ascending order with the help of any built-in function?

**Candidate:** : Sure, I can use the `sort` function to achieve that. Here's an example:

```javascript
var arr = [5, 2, 8, 1, 3];
arr.sort(function(a, b) {
  return a - b;
});
console.log(arr); // Output: [1, 2, 3, 5, 8]
```

**Interviewer:** : Great. What are the differences between `let` and `const`?

**Candidate:** : The main differences between `let` and `const` are that `let` has a block scope and can be reassigned, while `const` has a block scope and cannot be reassigned.

**Interviewer:** : Can you explain the concept of closures in JavaScript?

**Candidate:** : Certainly. Closures in JavaScript occur when a function is defined inside another function, allowing the inner function to access the outer function's variables. The inner function forms a closure, preserving the variables of the outer function even after it has finished executing.

**Interviewer:** : Good explanation. Now, what is the `this` keyword in JavaScript?

**Candidate:** : The `this` keyword refers to the object to which a function or method belongs. Its value is determined by how a function is called. In a global context, `this` refers to the global object, while in a method, `this` refers to the object that owns the method.

**Interviewer:** : Well explained. What is a promise in JavaScript?

**Candidate:** : A promise is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value. It helps in organizing asynchronous code and avoiding callback hell. A promise can be in three states: pending, fulfilled, or rejected.

**Interviewer:** : Can you explain the event loop in JavaScript?

**Candidate:** : Certainly. The event loop is a fundamental concept in JavaScript that manages the execution of code in a non-blocking way. It consists of the call stack, web APIs provided by the browser, a message queue, and the event loop itself. It ensures that asynchronous tasks are handled without blocking the main thread.

**Interviewer:** : Good overview. Lastly, what is the difference between synchronous and asynchronous code?

**Candidate:** : Synchronous code is executed sequentially, and each operation waits for the previous one to complete. Asynchronous code allows operations to be executed independently, and the program doesn't wait for each operation to finish before moving on to the next one. It's like waiting in a queue for synchronous and placing an order at a restaurant for asynchronous.