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Top 100 JSON Interview Questions and Answers

Top 100 JSON Interview Questions and Answers

Contents show

Question 1: What is JSON?

Answer: JSON (JavaScript Object Notation) is a lightweight data-interchange format that is easy for humans to read and write, and easy for machines to parse and generate.

Code Explanation:

{
  "name": "John Doe",
  "age": 30,
  "city": "New York"
}

Reference


Question 2: How is JSON different from XML?

Answer: JSON is more concise and easier to read than XML. It uses key-value pairs, while XML uses tags. JSON is typically faster to parse, making it more efficient for data transmission.

Code Explanation:

{
  "name": "John Doe",
  "age": 30
}

Reference


Question 3: How do you parse JSON in JavaScript?

Answer: In JavaScript, you can parse JSON using the JSON.parse() method.

Code Explanation:

const jsonString = '{"name": "John Doe", "age": 30}';
const obj = JSON.parse(jsonString);

Reference


Question 4: How do you convert an object to JSON in JavaScript?

Answer: In JavaScript, you can convert an object to JSON using the JSON.stringify() method.

Code Explanation:

const obj = { name: "John Doe", age: 30 };
const jsonString = JSON.stringify(obj);

Reference


Question 5: What is a JSON Schema?

Answer: A JSON Schema is a JSON document that defines the structure and constraints of other JSON documents. It acts as a blueprint for validating JSON data.

Code Explanation:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "number" }
  },
  "required": ["name", "age"]
}

Reference


Question 6: How do you validate JSON against a JSON Schema?

Answer: You can use libraries like AJV (Another JSON Schema Validator) in JavaScript to validate JSON against a JSON Schema.

Code Explanation:

const Ajv = require("ajv");
const ajv = new Ajv();
const schema = {
  type: "object",
  properties: {
    name: { type: "string" },
    age: { type: "number" }
  },
  required: ["name", "age"]
};
const validate = ajv.compile(schema);
const data = { name: "John Doe", age: 30 };
const isValid = validate(data);

Reference


Question 7: What is JSON Web Token (JWT)?

Answer: JSON Web Token is a compact, URL-safe means of representing claims to be transferred between two parties. It is often used for authentication and information exchange.

Reference


Question 8: How do you decode a JWT token?

Answer: In JavaScript, you can use libraries like jsonwebtoken to decode a JWT token.

Code Explanation:

const jwt = require('jsonwebtoken');
const token = 'YOUR_JWT_TOKEN_HERE';
const decoded = jwt.decode(token);

Reference


Question 9: What is JSONP?

Answer: JSONP (JSON with Padding) is a method to request data from a server in a different domain. It allows cross-domain requests in web pages.

Code Explanation:

function handleResponse(data) {
  console.log(data);
}

const script = document.createElement('script');
script.src = 'http://example.com/data?callback=handleResponse';
document.body.appendChild(script);

Reference


Question 10: What is JSON.stringify() replacer parameter?

Answer: The replacer parameter in JSON.stringify() is a function that alters the behavior of the stringification process, or an array of strings and numbers to select specific properties to include.

Code Explanation:

const obj = { name: "John Doe", age: 30, city: "New York" };
const jsonString = JSON.stringify(obj, ['name', 'age']);

Reference


Question 11: What is JSON.parse() reviver parameter?

Answer: The reviver parameter in JSON.parse() is an optional function that can filter and transform the results. It allows you to customize the process of turning the JSON data back into an object.

Code Explanation:

const json = '{"p": 5}';
const obj = JSON.parse(json, (key, value) => {
  if (key === '') return value; // root value, return as is
  return value * 2; // double all other values
});

Reference


Question 12: What is the MIME type for JSON?

Answer: The MIME type for JSON is application/json. It is used in HTTP headers to indicate that the content is JSON.

Reference


Question 13: How do you pretty-print JSON in JavaScript?

Answer: You can use the JSON.stringify() method with an additional parameter, space, to add indentation for better readability.

Code Explanation:

const obj = { name: "John Doe", age: 30, city: "New York" };
const jsonString = JSON.stringify(obj, null, 2); // 2 spaces for indentation

Reference


Question 14: What is JSONPath?

Answer: JSONPath is a query language that allows you to easily extract specific data from a JSON document. It’s similar to XPath for XML.

Code Explanation:

{
  "store": {
    "book": [
      {
        "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century"
      },
      {
        "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour"
      }
    ]
  }
}

JSONPath: $.store.book[0].title returns "Sayings of the Century"

Reference


Question 15: How do you remove a property from a JSON object?

Answer: In JavaScript, you can use the delete keyword to remove a property from a JSON object.

Code Explanation:

const obj = { name: "John Doe", age: 30, city: "New York" };
delete obj.city;

Reference


Question 16: What is JSONB in PostgreSQL?

Answer: JSONB is a data type in PostgreSQL that stores JSON data in a binary format. It allows for efficient storage and querying of JSON data.

Code Explanation:

CREATE TABLE users (
  id serial PRIMARY KEY,
  data jsonb
);

Reference


Question 17: How do you validate JSON syntax?

Answer: You can use online tools like JSONLint or built-in methods in programming languages (e.g., JSON.parse() in JavaScript) to validate JSON syntax.

Reference


Question 18: What is BSON?

Answer: BSON (Binary JSON) is a binary-encoded serialization of JSON-like documents. It’s designed to be more efficient in storage and data exchange.

Reference


Question 19: What is JSON-RPC?

Answer: JSON-RPC is a remote procedure call (RPC) protocol encoded in JSON. It allows for calling functions on a remote server as if they were local.

Code Explanation:

// Request
{
  "jsonrpc": "2.0",
  "method": "subtract",
  "params": [42, 23],
  "id": 1
}

// Response
{
  "jsonrpc": "2.0",
  "result": 19,
  "id": 1
}

Reference


Question 20: How do you handle errors in JSON-RPC?

Answer: In JSON-RPC, errors are handled by including an error property in the response object.

Code Explanation:

// Error Response
{
  "jsonrpc": "2.0",
  "error": {
    "code": -32601,
    "message": "Method not found"
  },
  "id": 1
}

Reference


Question 21: What is JSON Web Token (JWT)?

Answer: JWT is an open standard (RFC 7519) that defines a compact and self-contained way for securely transmitting information between parties as a JSON object.

Reference


Question 22: How do you encode and decode JWT in JavaScript?

Answer: In JavaScript, you can use libraries like jsonwebtoken to encode and decode JWTs.

Code Explanation:

const jwt = require('jsonwebtoken');

// Encoding
const token = jwt.sign({ data: 'example' }, 'secret', { expiresIn: '1h' });

// Decoding
const decoded = jwt.verify(token, 'secret');

Reference


Question 23: What is JSON Schema?

Answer: JSON Schema is a vocabulary that allows you to validate, annotate, and manipulate JSON documents.

Code Explanation:

{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "number" }
  },
  "required": ["name", "age"]
}

Reference


Question 24: What is JSONP?

Answer: JSONP (JSON with Padding) is a technique for requesting data from a server in a different domain. It allows you to bypass the same-origin policy.

Code Explanation:

function handleJSONP(data) {
  console.log(data);
}

const script = document.createElement('script');
script.src = 'https://example.com/data?callback=handleJSONP';
document.body.appendChild(script);

Reference


Question 25: How do you handle errors in JSONP?

Answer: JSONP doesn’t have a built-in error handling mechanism. You can implement a timeout function to handle timeouts or use other techniques like monitoring for specific callback names.

Code Explanation:

const timeout = setTimeout(() => {
  console.error('JSONP request timed out');
  document.body.removeChild(script);
}, 5000); // 5 seconds

// Inside handleJSONP function
clearTimeout(timeout);

Reference


Question 26: What is JSON-LD?

Answer: JSON-LD (JSON Linked Data) is a lightweight Linked Data format built on top of JSON. It is easy to write and easy to parse.

Code Explanation:

{
  "@context": "http://schema.org/",
  "@type": "Person",
  "name": "John Doe",
  "jobTitle": "Software Engineer"
}

Reference


Question 27: How do you parse JSON in Python?

Answer: In Python, you can use the json module to parse JSON.

Code Explanation:

import json

# JSON string
json_str = '{"name": "John", "age": 30}'
data = json.loads(json_str)

Reference


Question 28: What is the difference between JSON and BSON?

Answer: JSON is a text-based format for data exchange. BSON, on the other hand, is a binary-encoded serialization of JSON-like documents, designed for more efficient storage and data exchange.

Reference


Question 29: How can you pretty-print JSON in JavaScript?

Answer: You can use JSON.stringify() with optional parameters to pretty-print JSON in JavaScript.

Code Explanation:

const data = { name: 'John', age: 30 };
const prettyJSON = JSON.stringify(data, null, 2); // 2 is the number of spaces for indentation

Reference


Question 30: What is JSON-RPC?

Answer: JSON-RPC is a remote procedure call (RPC) protocol encoded in JSON. It allows for communication between a client and a server using JSON.

Code Explanation:

// Request
{
  "jsonrpc": "2.0",
  "method": "subtract",
  "params": [42, 23],
  "id": 1
}

// Response
{
  "jsonrpc": "2.0",
  "result": 19,
  "id": 1
}

Reference


Question 31: What is JSONata?

Answer: JSONata is a lightweight query and transformation language for JSON data.

Code Explanation:

// JSONata expression
$.orders[0].items[?(price > 9.99)].name

Reference


Question 32: How can you validate JSON in JavaScript?

Answer: You can use JSON.parse() and wrap it in a try-catch block to validate JSON.

Code Explanation:

try {
  const valid = JSON.parse(jsonString);
  console.log('JSON is valid');
} catch (e) {
  console.error('Invalid JSON');
}

Reference


Question 33: What is the difference between XML and JSON?

Answer: XML is a markup language for defining custom document types, whereas JSON is a lightweight data interchange format. JSON is generally more readable and easier to parse than XML.

Reference


Question 34: How do you convert JSON to a JavaScript object?

Answer: You can use JSON.parse() to convert JSON to a JavaScript object.

Code Explanation:

const jsonString = '{"name": "John", "age": 30}';
const obj = JSON.parse(jsonString);

Reference


Question 35: How do you convert a JavaScript object to JSON?

Answer: You can use JSON.stringify() to convert a JavaScript object to JSON.

Code Explanation:

const obj = { name: 'John', age: 30 };
const jsonString = JSON.stringify(obj);

Reference


Question 36: What is JSON Web Encryption (JWE)?

Answer: JSON Web Encryption (JWE) is a standard for encrypting JSON-based data. It provides a way to securely transmit information between parties.

Reference


Question 37: How do you handle circular references in JSON?

Answer: In JavaScript, you can use a library like flatted or implement a custom serialization function to handle circular references.

Code Explanation:

const CircularJSON = require('flatted');
const obj = { a: 1 };
obj.b = obj;
const jsonString = CircularJSON.stringify(obj);
const parsedObj = CircularJSON.parse(jsonString);

Reference


Question 38: What is JSON Lines (JSONL)?

Answer: JSON Lines (JSONL) is a text format for storing structured data where each line is a valid JSON value.

Code Explanation:

// Example JSONL file
{"name": "John", "age": 30}
{"name": "Jane", "age": 25}

Reference


Question 39: How do you merge two JSON objects in JavaScript?

Answer: You can use the spread operator (...) to merge two JSON objects in JavaScript.

Code Explanation:

const obj1 = { a: 1, b: 2 };
const obj2 = { b: 3, c: 4 };
const mergedObj = { ...obj1, ...obj2 };

Reference


Question 40: What is JSON Merge Patch?

Answer: JSON Merge Patch is a format for describing changes to a JSON document. It’s designed to be simple and efficient for partial updates.

Code Explanation:

// Merge Patch document
{
  "title": "New Title",
  "author": {
    "name": "Jane Doe"
  }
}

Reference


Question 41: How do you escape special characters in JSON?

Answer: In JSON, you can escape special characters using backslashes (\). For example, \" represents a double quote.

Code Explanation:

{
  "message": "This is a \"quoted\" string."
}

Reference


Question 42: What is JSON Schema?

Answer: JSON Schema is a vocabulary that allows you to annotate and validate JSON documents. It provides a way to define the structure, content, and format of JSON data.

Code Explanation:

// Example JSON Schema
{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "number" }
  },
  "required": ["name", "age"]
}

Reference


Question 43: How do you perform deep cloning of a JSON object in JavaScript?

Answer: You can use JSON.parse(JSON.stringify(obj)) to perform a deep clone of a JSON object in JavaScript.

Code Explanation:

const originalObj = { a: 1, b: { c: 2 } };
const clonedObj = JSON.parse(JSON.stringify(originalObj));

Reference


Question 44: What is JSON Feed?

Answer: JSON Feed is a lightweight format for presenting RSS/Atom feeds in JSON. It aims to be simpler and easier to read than XML-based formats.

Code Explanation:

// Example JSON Feed
{
  "version": "https://jsonfeed.org/version/1",
  "title": "Sample Feed",
  "items": [
    {
      "id": "1",
      "title": "Post 1",
      "content_html": "<p>Content for Post 1</p>"
    }
  ]
}

Reference


Question 45: How can you sort JSON objects by a specific key in JavaScript?

Answer: You can use the sort() method with a custom comparison function to sort JSON objects by a specific key in JavaScript.

Code Explanation:

const data = [
  { name: 'John', age: 30 },
  { name: 'Jane', age: 25 },
];

data.sort((a, b) => a.age - b.age); // Sort by age

Reference


Question 46: What is JSON Web Token (JWT)?

Answer: JSON Web Token (JWT) is an open standard for securely transmitting information between parties as a JSON object. It is commonly used for authentication and information exchange.

Reference


Question 47: How do you validate a JSON string against a JSON Schema in JavaScript?

Answer: You can use libraries like ajv or jsonschema in JavaScript to validate a JSON string against a JSON Schema.

Code Explanation:

const Ajv = require('ajv');
const ajv = new Ajv();
const schema = {
  type: 'object',
  properties: {
    name: { type: 'string' },
    age: { type: 'number' }
  },
  required: ['name', 'age']
};
const validate = ajv.compile(schema);
const data = { name: 'John', age: 30 };
const valid = validate(data);

Reference


Question 48: What is JSON-RPC?

Answer: JSON-RPC is a remote procedure call (RPC) protocol encoded in JSON. It allows for communication between a client and a server over a network.

Code Explanation:

// Example JSON-RPC Request
{
  "jsonrpc": "2.0",
  "method": "subtract",
  "params": [42, 23],
  "id": 1
}

Reference


Question 49: How do you pretty-print JSON in JavaScript?

Answer: You can use JSON.stringify(obj, null, 2) with the space parameter set to 2 to pretty-print JSON in JavaScript.

Code Explanation:

const obj = { name: 'John', age: 30 };
const jsonString = JSON.stringify(obj, null, 2);

Reference


Question 50: What is JSON-B?

Answer: JSON-B is a standard binding for converting Java objects to JSON and vice versa. It provides a convenient way to work with JSON in Java applications.

Code Explanation:

// Example JSON-B usage in Java
import javax.json.bind.Jsonb;
import javax.json.bind.JsonbBuilder;

Jsonb jsonb = JsonbBuilder.create();
String jsonString = jsonb.toJson(person);

Reference


Question 51: What is JSON Pointer?

Answer: JSON Pointer is a string syntax used to reference a specific value within a JSON document. It is commonly used in applications that need to identify a specific part of a JSON structure.

Code Explanation:

// Example JSON Pointer
{
  "user": {
    "name": "John Doe",
    "address": {
      "city": "New York",
      "postalCode": "10001"
    }
  }
}

The JSON Pointer "/user/address/city" would reference the value "New York".

Reference


Question 52: How do you handle errors in JSON parsing in JavaScript?

Answer: You can use a try-catch block to handle errors in JSON parsing in JavaScript.

Code Explanation:

try {
  const json = JSON.parse('invalid JSON');
} catch (error) {
  console.error('Error:', error.message);
}

Reference


Question 53: What is JSON Merge Patch?

Answer: JSON Merge Patch is a format for describing changes to a JSON document. It is often used in applications that need to update or patch JSON data.

Code Explanation:

// Example JSON Merge Patch
{
  "title": "New Title",
  "author": {
    "name": "Jane Doe"
  }
}

Reference


Question 54: How can you convert a JSON object to a URL-encoded string in JavaScript?

Answer: You can use encodeURIComponent(JSON.stringify(obj)) to convert a JSON object to a URL-encoded string in JavaScript.

Code Explanation:

const obj = { name: 'John Doe', age: 30 };
const urlEncodedString = encodeURIComponent(JSON.stringify(obj));

Reference


Question 55: What is JSON Lines?

Answer: JSON Lines (JSONL) is a text format for storing structured data where each line is a valid JSON object. It is commonly used for processing large datasets.

Code Explanation:

// Example JSON Lines
{"name": "John Doe", "age": 30}
{"name": "Jane Doe", "age": 25}

Reference


Question 56: How do you parse JSON in Python?

Answer: In Python, you can use the json module to parse JSON data. The json.loads() function is used to deserialize a JSON string.

Code Explanation:

import json

json_string = '{"name": "John Doe", "age": 30}'
json_data = json.loads(json_string)

Reference


Question 57: What is JSON Web Token (JWT)?

Answer: JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. It is often used for authentication and information exchange in web development.

Code Explanation:

// Example JWT
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

Reference


Question 58: How do you generate a random JSON Web Token (JWT) in Node.js?

Answer: You can use libraries like jsonwebtoken in Node.js to generate a random JWT.

Code Explanation:

const jwt = require('jsonwebtoken');

const secretKey = 'mySecretKey';
const payload = { userId: 12345, username: 'john_doe' };
const token = jwt.sign(payload, secretKey);

Reference


Question 59: What is JSON Schema?

Answer: JSON Schema is a vocabulary that allows you to validate, annotate, and manipulate JSON documents. It provides a set of rules to define the structure of valid JSON data.

Code Explanation:

// Example JSON Schema
{
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "age": { "type": "number" }
  },
  "required": ["name", "age"]
}

Reference


Question 60: How do you validate a JSON string against a JSON Schema in Python?

Answer: You can use libraries like jsonschema in Python to validate a JSON string against a JSON Schema.

Code Explanation:

import jsonschema

schema = {
    "type": "object",
    "properties": {
        "name": {"type": "string"},
        "age": {"type": "number"}
    },
    "required": ["name", "age"]
}

data = {"name": "John Doe", "age": 30}

jsonschema.validate(data, schema)

Reference


Question 61: What is the purpose of the Content-Type header in HTTP requests?

Answer: The Content-Type header specifies the type of data being sent in the HTTP request or response. For example, when sending JSON data, the Content-Type header is set to application/json.

Code Explanation:

Content-Type: application/json

Reference


Question 62: How do you set the Content-Type header in an Axios HTTP request?

Answer: In Axios, you can set the Content-Type header using the headers property in the request configuration.

Code Explanation:

axios.post('/api/endpoint', data, {
  headers: {
    'Content-Type': 'application/json'
  }
});

Reference


Question 63: What is Cross-Origin Resource Sharing (CORS) and how does it work?

Answer: CORS is a security feature implemented by web browsers that restricts web pages from making requests to a different domain than the one that served the web page. It works by adding HTTP headers to responses, indicating which origins are allowed to access the resources.

Code Explanation:

Access-Control-Allow-Origin: https://example.com

Reference


Question 64: How do you enable CORS in a Node.js application?

Answer: You can enable CORS in a Node.js application using middleware like cors.

Code Explanation:

const express = require('express');
const cors = require('cors');

const app = express();
app.use(cors());

Reference


Question 65: What is the purpose of HTTP cookies?

Answer: HTTP cookies are small pieces of data stored in a user’s web browser. They are used to remember information about the user, such as login status, preferences, and shopping cart contents.

Code Explanation:

Set-Cookie: sessionId=abc123; Path=/; Expires=Wed, 09 Jun 2021 10:18:14 GMT

Reference


Question 66: How do you set an HTTP cookie in a server response?

Answer: You can set an HTTP cookie in a server response by including a Set-Cookie header.

Code Explanation:

Set-Cookie: sessionId=abc123; Path=/; Expires=Wed, 09 Jun 2021 10:18:14 GMT

Reference


Question 67: How do you send an HTTP request with a cookie using Axios?

Answer: To send an HTTP request with a cookie using Axios, you can include the cookie in the headers property of the request configuration.

Code Explanation:

axios.get('/api/endpoint', {
  headers: {
    Cookie: 'sessionId=abc123'
  }
});

Reference


Question 68: What is a JSON Web Token (JWT) and how is it used for authentication?

Answer: A JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. It is often used for authentication and information exchange in web development.

Code Explanation:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

Reference


Question 69: How do you decode a JWT in Node.js?

Answer: To decode a JWT in Node.js, you can use libraries like jsonwebtoken.

Code Explanation:

const jwt = require('jsonwebtoken');
const decoded = jwt.decode(token);

Reference


Question 70: What is OAuth and how does it work?

Answer: OAuth is an open standard for access delegation, commonly used for token-based authentication. It allows a user to grant a third-party application limited access to their resources without exposing their credentials.

Code Explanation:

GET /authorize?client_id=CLIENT_ID&redirect_uri=REDIRECT_URI&response_type=code

Reference


Question 71: How do you generate a random string in JavaScript?

Answer: You can generate a random string in JavaScript using the Math.random() function along with string manipulation methods.

Code Explanation:

const randomString = Math.random().toString(36).substring(2, 15);

Reference


Question 72: What is the purpose of a nonce in OAuth?

Answer: A nonce (number used once) is a random or pseudo-random value that is used only once in a cryptographic communication. In OAuth, it is used to prevent replay attacks.

Code Explanation:

oauth_nonce=7d8f3e4a

Reference


Question 73: How do you handle CORS (Cross-Origin Resource Sharing) in a Node.js application?

Answer: To handle CORS in a Node.js application, you can use the cors middleware. This middleware adds the necessary headers to allow or deny cross-origin requests.

Code Explanation:

const cors = require('cors');
const express = require('express');
const app = express();

app.use(cors());

Reference


Question 74: How can you prevent SQL injection attacks in a Node.js application?

Answer: To prevent SQL injection attacks, you should use parameterized queries or prepared statements. Libraries like mysql2 provide support for prepared statements in Node.js.

Code Explanation:

const mysql = require('mysql2');

const connection = mysql.createConnection({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'mydatabase'
});

const query = 'SELECT * FROM users WHERE username = ?';
connection.execute(query, ['user_input'], (error, results) => {
  // Handle results
});

Reference


Question 75: What is a Promise in JavaScript and how does it work?

Answer: A Promise in JavaScript represents the eventual completion or failure of an asynchronous operation. It allows you to write asynchronous code in a more synchronous manner.

Code Explanation:

const myPromise = new Promise((resolve, reject) => {
  // Async operation
  if (success) {
    resolve('Success!');
  } else {
    reject('Error!');
  }
});

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

Reference


Question 76: How do you upload a file in a Node.js application?

Answer: You can handle file uploads in a Node.js application using libraries like multer which provides middleware for handling multipart/form-data.

Code Explanation:

const multer  = require('multer');
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), (req, res, next) => {
  // Handle uploaded file
});

Reference


Question 77: Explain the difference between let, const, and var in JavaScript.

Answer: let and const were introduced in ES6. let allows you to declare variables with block scope, while const creates a constant (unchangeable) reference to a value. var has function scope and is function-scoped.

Code Explanation:

let x = 10;
const y = 20;
var z = 30;

Reference


Question 78: How can you handle authentication in a Node.js application?

Answer: Authentication in a Node.js application can be handled using libraries like Passport.js. It provides a middleware for authenticating requests.

Code Explanation:

const passport = require('passport');

app.post('/login', passport.authenticate('local', {
  successRedirect: '/dashboard',
  failureRedirect: '/login'
}));

Reference


Question 79: What is npm and what is its role in Node.js?

Answer: npm (Node Package Manager) is the default package manager for Node.js. It allows developers to easily manage and install external libraries or packages needed for a project.

Code Explanation:

# Installing a package using npm
npm install package_name

Reference


Question 80: How can you handle sessions in a Node.js application?

Answer: You can handle sessions in a Node.js application using middleware like express-session. It creates a session object which can be used to store data across multiple requests.

Code Explanation:

const session = require('express-session');

app.use(session({
  secret: 'secret_key',
  resave: false,
  saveUninitialized: true
}));

Reference


Question 81: Explain the concept of middleware in Express.js.

Answer: Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. They can perform tasks, modify req and res objects, and end the request-response cycle.

Code Explanation:

app.use((req, res, next) => {
  // Do something before sending the response
  next(); // Calls the next middleware function
});

Reference


Question 82: What is the purpose of the package.json file in a Node.js application?

Answer: The package.json file contains metadata about the project and its dependencies. It includes information like the project name, version, description, and a list of dependencies required to run the project.

Code Explanation:

{
  "name": "my_project",
  "version": "1.0.0",
  "description": "My Node.js project",
  "dependencies": {
    "express": "^4.17.1",
    "mysql": "^2.18.1"
  }
}

Reference


Question 83: What is the purpose of the package-lock.json file?

Answer: The package-lock.json file is automatically generated by npm to lock down the version of dependencies used in a project. It ensures that the same versions of packages are installed across different environments, providing consistency.

Code Explanation:

The package-lock.json file is automatically generated by npm.

Reference


Question 84: How can you handle file uploads in a Node.js application?

Answer: You can handle file uploads in a Node.js application using middleware like multer. Multer is a node.js middleware for handling multipart/form-data, which is primarily used for uploading files.

Code Explanation:

const multer = require('multer');
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), (req, res) => {
  // Access uploaded file using req.file
});

Reference


Question 85: Explain the purpose of the Node.js util module.

Answer: The util module provides a set of utility functions that are useful for various tasks. It includes functions for inheritance, error handling, and formatting.

Code Explanation:

const util = require('util');
const throwError = util.promisify((msg, cb) => cb(new Error(msg)));

throwError('An error occurred')
  .catch(err => console.error(err.message));

Reference


Question 86: What is the purpose of the child_process module in Node.js?

Answer: The child_process module allows Node.js to spawn and interact with child processes. This is useful for running external commands or scripts, allowing the Node.js application to perform tasks in parallel.

Code Explanation:

const { exec } = require('child_process');

exec('ls -l', (err, stdout, stderr) => {
  if (err) {
    console.error(`Error: ${err}`);
    return;
  }
  console.log(`Output: ${stdout}`);
});

Reference


Question 87: How can you handle environment variables in a Node.js application?

Answer: You can handle environment variables in a Node.js application using the process.env object. This allows you to access variables set in the environment or pass configuration values to your application.

Code Explanation:

const port = process.env.PORT || 3000;

Reference


Question 88: Explain the purpose of the os module in Node.js.

Answer: The os module provides a set of operating system-related utility methods. It allows you to retrieve information about the system such as CPU architecture, memory, network interfaces, and more.

Code Explanation:

const os = require('os');

console.log(`Total Memory: ${os.totalmem()}`);
console.log(`Free Memory: ${os.freemem()}`);

Reference


Question 89: How can you perform asynchronous operations in Node.js?

Answer: Asynchronous operations in Node.js can be achieved using callbacks, Promises, or the async/await syntax. Callbacks are the traditional way, Promises provide a more structured approach, and async/await offers a cleaner syntax for working with asynchronous code.

Code Explanation:

Using Promises:

const fs = require('fs');

const readFilePromise = (path) => {
  return new Promise((resolve, reject) => {
    fs.readFile(path, (err, data) => {
      if (err) reject(err);
      else resolve(data);
    });
  });
};

readFilePromise('file.txt')
  .then(data => console.log(data.toString()))
  .catch(err => console.error(err));

Using async/await:

const fs = require('fs').promises;

const readFileAsync = async (path) => {
  try {
    const data = await fs.readFile(path);
    console.log(data.toString());
  } catch (err) {
    console.error(err);
  }
};

readFileAsync('file.txt');

Reference


Question 90: What is the purpose of the package.json file in a Node.js project?

Answer: The package.json file is a crucial part of Node.js projects. It contains metadata about the project, including dependencies, scripts, and other important information. It’s used to manage project dependencies, define scripts for tasks like testing or running the application, and store key information about the project.

Code Explanation:

Here’s a basic example of a package.json file:

{
  "name": "my-app",
  "version": "1.0.0",
  "description": "A sample Node.js application",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "test": "mocha tests/*.js"
  },
  "dependencies": {
    "express": "^4.17.1",
    "lodash": "^4.17.21"
  },
  "devDependencies": {
    "mocha": "^9.0.0"
  },
  "author": "Your Name",
  "license": "MIT"
}

Explanation:

  • name: Specifies the name of the project.
  • version: Defines the version of the project.
  • description: Provides a short description of the project.
  • main: Specifies the entry point of the application.
  • scripts: Contains a set of predefined scripts that can be executed with npm run.
  • dependencies: Lists the production dependencies required for the project to run.
  • devDependencies: Lists dependencies needed for development, like testing libraries.
  • author: Specifies the author of the project.
  • license: Declares the type of license under which the project is distributed.

Reference


Question 91: Explain the concept of middleware in Express.js.

Answer: Middleware in Express.js are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. They can perform various tasks such as modifying the request and response objects, ending the request-response cycle, and calling the next middleware function.

Code Explanation:

Here’s an example of a simple middleware function that logs the request URL:

function logRequest(req, res, next) {
  console.log(`Received a ${req.method} request for ${req.url}`);
  next(); // Call the next middleware function
}

app.use(logRequest); // Register the middleware

Explanation:

  • The logRequest function takes three parameters: req (request object), res (response object), and next (next middleware function).
  • It logs the method and URL of the incoming request.
  • next() is called to pass control to the next middleware function in the stack.

Reference:

Express.js Middleware


Question 92: How can you handle errors in an Express.js application?

Answer: Errors in Express.js can be handled using middleware functions. These are special functions with four parameters, where the first one (err) represents the error object. If an error occurs, it should be passed to the next() function to trigger the error-handling middleware.

Code Explanation:

Here’s an example of an error-handling middleware:

function errorHandler(err, req, res, next) {
  res.status(500).json({ error: 'Internal Server Error' });
}

app.use(errorHandler); // Register the error-handling middleware

Explanation:

  • The errorHandler function takes four parameters, with the first one (err) representing the error object.
  • It responds with a 500 status code and a JSON object containing an error message.

Reference:

Error Handling in Express.js


Question 93: What is CSRF protection and how can it be implemented in an Express.js application?

Answer: Cross-Site Request Forgery (CSRF) is an attack where a malicious website tricks a user’s browser into making a request to a different site on which the user is authenticated. To protect against this, Express.js applications can use CSRF tokens.

Code Explanation:

To implement CSRF protection, you can use the csurf middleware:

const csrf = require('csurf');
const csrfProtection = csrf({ cookie: true });

// Apply CSRF protection to routes
app.use(csrfProtection);

// In a route handler, generate a token and pass it to the view
app.get('/some-route', (req, res) => {
  const token = req.csrfToken();
  res.render('some-template', { csrfToken: token });
});

Explanation:

  • csurf is a package for CSRF protection in Express.js.
  • It generates a CSRF token and attaches it to the request object (req.csrfToken()).
  • The token should be included in forms or AJAX requests and validated on the server side.

Reference:

csurf npm package


Question 94: What is a Promise in JavaScript? How can you use Promises in an Express.js application?

Answer: A Promise in JavaScript represents the eventual completion or failure of an asynchronous operation. It allows you to work with asynchronous code in a more readable and organized manner.

Code Explanation:

Using Promises in an Express.js route handler:

app.get('/some-route', (req, res) => {
  new Promise((resolve, reject) => {
    // Asynchronous operation
    if (/* operation successful */) {
      resolve('Success');
    } else {
      reject(new Error('Error'));
    }
  })
  .then(data => {
    res.send(data);
  })
  .catch(error => {
    res.status(500).send(error.message);
  });
});

Explanation:

  • In the route handler, a Promise is created with an asynchronous operation inside it.
  • If the operation is successful, it calls resolve with the result. If there’s an error, it calls reject with an error object.
  • .then() is used to handle the resolved value, and .catch() is used to handle errors.

Reference:

MDN Web Docs – Promises


Question 95: How can you serve static files in Express.js?

Answer: You can use the express.static middleware to serve static files, such as HTML, CSS, images, and JavaScript, from a directory.

Code Explanation:

app.use(express.static('public'));

Explanation:

  • In this example, the public directory is specified as the source of static files.
  • Files in the public directory can be accessed directly from the browser.

Reference:

Express.js – Serving Static Files


Question 96: What is routing in Express.js?

Answer: Routing in Express.js refers to the process of defining the end points (URIs) at which an application responds to client requests. Each route can have a specific request method (GET, POST, etc.) and a callback function to handle the request.

Code Explanation:

app.get('/some-route', (req, res) => {
  res.send('Hello, World!');
});

Explanation:

  • This code defines a route using app.get().
  • When a GET request is made to /some-route, the callback function is executed, sending the response “Hello, World!”.

Reference:

Express.js – Routing


Question 97: How can you handle errors in an Express.js application?

Answer: Express.js provides middleware for error handling. You can create an error handling middleware function that takes four arguments (err, req, res, next) and use it after all other routes.

Code Explanation:

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something went wrong!');
});

Explanation:

  • The middleware takes an error object err, the request req, the response res, and the next middleware next.
  • In this example, the error is logged and a generic error message is sent to the client with a status code of 500.

Reference:

Express.js – Error Handling


Question 98: What is middleware in Express.js?

Answer: Middleware in Express.js are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. They can perform tasks, modify the request and response objects, and end the request-response cycle.

Code Explanation:

const myMiddleware = (req, res, next) => {
  // Do something before sending the response
  next(); // Call the next middleware
};

app.use(myMiddleware);

Explanation:

  • myMiddleware is an example of a middleware function.
  • It takes the request, response, and the next middleware function as arguments.
  • It can execute code before the response is sent.

Reference:

Express.js – Middleware


Question 99: How can you handle form data in an Express.js application?

Answer: You can use the body-parser middleware to parse form data in an Express.js application.

Code Explanation:

const bodyParser = require('body-parser');

// Parse URL-encoded bodies (for form data)
app.use(bodyParser.urlencoded({ extended: true }));

app.post('/form-submit', (req, res) => {
  const formData = req.body;
  // Process the form data
});

Explanation:

  • body-parser is used to parse the request body.
  • The urlencoded option is set to true to parse extended URL-encoded bodies.
  • In the route handler, req.body contains the form data.

Reference:

body-parser npm package


Question 100: How can you secure an Express.js application?

Answer: To secure an Express.js application, you can implement various measures such as using HTTPS, validating user input, using secure authentication methods, setting proper headers, and implementing security middleware.

Code Explanation:

const helmet = require('helmet');
app.use(helmet());

Explanation:

  • helmet is a package that helps secure Express.js apps by setting various HTTP headers.
  • When applied as middleware, it automatically adds security headers.

Reference:

Helmet npm package