Express.js Interview Questions and Answers: A Comprehensive Guide

Ace your next Express.js interview with these meticulously crafted questions and answers, meticulously curated from the wisdom of two renowned resources: GeeksforGeeks and Simplilearn.

Express.js, a formidable force in the Node.js ecosystem, empowers developers to craft robust and efficient web applications. This comprehensive guide delves into the depths of Express.js equipping you with the knowledge to conquer your next interview.

1. What is Expressjs?

Express. js, a minimalist yet mighty framework, sits atop Node. js, simplifying its API and enriching it with valuable features. It streamlines application organization through middleware and routing, augments Node. js comes with useful utilities for HTTP objects and makes it easier to render dynamic HTTP objects

2. Why Use Express.js?

Expressjs, a beacon of simplicity, minimalism, flexibility, and scalability, empowers developers to construct server-side web applications and APIs with unparalleled efficiency Its straightforward setup for middlewares and routing makes it an indispensable tool for modern web development.

3. Write a ‘Hello World’ Express.js Application

To embark on your Express.js journey first install Express.js within your Node.js application using the following command

npm install express

Next navigate to your app.js file and craft the following code

const express = require('express');const app = express();const PORT = 8000;app.get('/', (req, res) => {  res.send('Hello World!');});app.listen(PORT, () => {  console.log(`Server is listening at port :${PORT}`);});

4. Differentiate Between Node.js and Express.js

Node. js, a runtime environment, empowers you to execute JavaScript on the server side. Express. js, on the other hand, is a framework built upon Node. js, providing a comprehensive set of tools for crafting web applications and APIs.

5. Is Express.js a Front-End or Back-End Framework?

Express. js, a cornerstone of the MEAN stack, is a JavaScript back-end framework. Its primary purpose is to develop comprehensive web applications and APIs.

6. Mention a Few Features of Express.js

Express.js boasts an array of impressive features, including:

• Routing: Express.js provides a straightforward approach to defining routes for handling HTTP requests. These routes map distinct URLs to specific code segments, streamlining application logic organization.
• Middleware: Express.js leverages middleware functions to execute tasks during the request-response cycle. These functions have access to the request, response, and the subsequent middleware function.
• HTTP Utility Methods: Express.js is primarily used for handling HTTP methods like GET, POST, PUT, and DELETE. This simplifies the process of defining how the application should respond to various HTTP requests.
• Static File Serving: With the help of the built-in express.static middleware, Express.js can serve static files, such as images, CSS, and JavaScript files.
• Security: Express.js incorporates features and middleware to enhance the security of your web applications, such as the helmet middleware to safeguard your app.

7. Explain the Structure of an Express.js Application

The structure of an Express.js application can vary depending on its complexity and project-specific requirements. However, a common approach involves the following elements:

• Entry point: This is the starting point of the application, where you set up your server, connect to your database, add middleware, and define the main routes.
• Routes directory: This directory houses the files for the app’s routes.
• Controllers directory: This directory contains files that define the logic for handling requests for a specific route.
• Models directory: This directory is used for creating the schema models for the different data.
• Middleware directory: This directory contains custom middleware functions that you can use in your routes.
• Views directory: If you’re using a templating engine, this directory contains your view templates.
• Public directory: This directory contains static files that are served directly by the server, such as images, CSS files, and JavaScript files.

This structure promotes separation of concerns in a logical way, making the application easier to understand and maintain.

8. What are Some Popular Alternatives to Express.js?

While Express.js reigns supreme, several other popular alternatives exist, including Koa.js, Hapi.js, Sails.js, and Fastify.

9. Which Major Tools Can Be Integrated with Express.js?

Express.js offers seamless integration with a plethora of tools and libraries, such as:

• Database tools: MongoDB, MySQL, PostgreSQL
• Template Engines: EJS, Pug, Mustache
• Authentication libraries: Passport.js
• Logging libraries: Morgan, Winston
• Validation libraries: Joi, express-validator
• ORM libraries: Sequelize, Mongoose

10. What is the .env File Used For?

The .env file serves as a secure storage for sensitive information within a web application, such as passwords and database connection strings. This simple text file stores key-value pairs that configure various aspects of the application.

11. What are JWTs?

JSON Web Tokens (JWTs) are tokens primarily used for authentication and information exchange. When a user signs in to an application, the application assigns a JWT to that user. Subsequent requests by the user will include the assigned JWT. This token informs the server about the routes, services, and resources the user is authorized to access. A JWT comprises three parts: Header, Payload, and Signature.

12. Create a Simple Middleware for Validating Users

// Simple user validation middlewareconst validateUser = (req, res, next) => {  const user = req.user;  // Check if the user object is present  if (!user) {    return res.status(401).json({ error: 'Unauthorized - User not found' });  }  // If the user is valid, move to the next middleware or route handler  next();};// Example of using the middleware in an Express routeapp.get('/profile', validateUser, (req, res) => {  const user = req.user;  res.json({ message: 'Profile page', username: user.username });});

13. What is Bcrypt Used For?

Bcrypt, a password hashing function, is used to securely hash and store user passwords. It is designed to be slow and computationally intensive, making it resistant to brute-force attacks and rainbow table attacks. Bcrypt is a key component in enhancing the security of user authentication systems.

14. Why Should You Separate the Express App and Server?

Separating the Express App and the server setup in Express.js is highly recommended. This approach offers modularity and flexibility, making the codebase more maintainable and testable. Here are some compelling reasons to embrace this separation:

• Modularity: You can define routes, middleware, and other components in the Express app independently of the server configuration.
• Ease of Testing: Separation makes it easier to write unit tests for the Express app without starting an actual server. You can test routes, middleware, and other components in isolation.
• Reusability: You can reuse the same Express app in different server configurations.
• Configuration Management: Separating the app and server allows for cleaner configuration management.
• Scalability: It provides a foundation for a scalable code structure. As your application grows, it will be easier to maintain the code.

15. What Do You Understand About ESLint?

ESLint, a JavaScript linting tool, automatically detects incorrect patterns found in ECMAScript/JavaScript code. Its purpose is to improve code quality, make code more consistent, and avoid bugs. ESLint is written using Node.js to provide a fast runtime environment and easy installation via npm.

16. Define the Concept of the Test Pyramid

The Test Pyramid, a concept in software testing, represents the distribution of different types of tests. Introduced by Mike Cohn, it suggests that a testing strategy should be shaped like a pyramid, with the majority of tests at the base and fewer tests as you move up. The Test Pyramid consists of three levels: Unit Tests, Integration Tests, and End-to-End (E2E) Tests.

17. Differentiate Between res.send() and res.json()

Both res.send() and res.json() serve similar purposes with a slight difference. The choice depends on the data type you’re working with. Opt for res.json() when dealing specifically with JSON data. Use res.send() when you need versatility and control over the content type or when handling various data types in your responses.

18. What is Meant by Scaffolding in Express.js?

Scaffolding in Express.js refers to the process of generating a basic project structure automatically. This can speed up the initial setup and help maintain consistency in the way projects are structured, especially in large teams.

19. How Would You Install an Express Application Generator for Scaffolding?

Express application generators are used to quickly set up a new Express application with some basic structure. You can install it using Node Package Manager (npm), which comes with Node.js.

To install it globally: