Top Express.js Interview Questions and Answers

In today’s world of web development tools, Express.js stands out as one of the most popular and used web application frameworks. You can get used to Express.js along with Node.js because it uses Node.js to build any web or mobile application. You can perform tasks like routing, middleware management and handling HTTP requests very easily and with minimal effort with the help of Express.js. Hence it is one of the most favourite tools among developers who look towards creating quick and efficient scalable applications.

In this blog, we will discuss all the important features of Exprees.js Interview questions from experienced developers as well as newbies. This will prepare you well for the interview and gives an idea of ​​what kind of questions are asked in such interviews. Overall, it will help you brush up on your skills and understanding of Express.js.

Express.js Interview Questions for Freshers

What is Express.js?

Express.js is one of the most used and easy-to-implement web application frameworks. This framework provides you with a lot of features for programming both the web and mobile frameworks. Overall, it simplifies building web servers and APIs by providing easy-to-use functions.

Why use Express.js?

Express.js is favoured for its simplicity, flexibility, and scalability. It provides a thin layer of fundamental web application features without obscuring Node.js features.

Differentiate between Node.js and Express.js?

Is Express JS a front-end or a back-end framework?

Express.js is used to manage the server-side logic of web applications. It is a backend framework that handles HTTP request routing, and middleware and works with the database. You also have access to features that serve data and files used by front-end technologies such as HTML, CSS, and JavaScript.

How would you install Express.js?

To install Express.js, first, you must have Node.js installed on your system, all along with npm. After that, you can use npm to install this framework using the following command.

npm install express

Write a ‘Hello World’ Express.js application.

Here, it shows a simple Expres.js application that is set to port 3000 on the local server.

What is the purpose of the app.use() function?

The app.use() function in Express.js is used to apply middleware to your application. For each incoming request, middleware jobs are executed sequentially. It performs various operations such as recording, parsing, and authentication. This method helps in modularising the code by separating different aspects. of request management

What are route handlers, and how would you implement them?

Route handlers in Express.js are functions that handle requests to specific endpoints in your application. It is defined in HTTP methods like GET, POST, PUT, DELETE etc. The basic usage is:

This code handles GET requests to the /example route and sends a response to the client.

What are Route-Handler chains?

Route-handler chains allow you to define multiple middleware functions that can handle a request before sending a final response. Each middleware in the chain can perform specific tasks like validation or logging and then pass control to the next function using the next() function, ensuring a clean separation of concerns.

How can we render a plain HTML?

In Express.js, you can render a plain HTML file using the res.sendFile() method. For example:

Now, this will allow you to send static HTML content as a response to a request because it serves the index.html file, which is located in the root directory of the project.

Mention a few features of Express.js.

• Middleware support: Allows using features that could manage requests and responses.
• Routing: Provides an easy manner to outline routes on your utility.
• Templating engines: Supports various templating engines like Pug and EJS.
• HTTP utility methods: Simplifies tasks like handling requests and responses.
• Static file serving: Easily serve static documents.
• Database integration: Works seamlessly with databases like MongoDB, MySQL, and PostgreSQL.

Write a code to post a query in Express.js.

This code sets up an Express.js server that listens for POST requests at the /submit route, processes the incoming JSON data, and sends a confirmation response.

Compare GET vs POST methods in Express.js.

What is the difference between a traditional server and an Express.js server?

A traditional server, like Apache or Nginx, is a totally-fledged internet server that can serve static content material, manipulate requests, and support various protocols which are out of the box. Express.js server, however, is a lightweight framework built on Node.js. It provides developers with the flexibility to define routes, handle HTTP requests, and serve dynamic content, but requires manual setup and configuration for many features provided by traditional servers.

Differentiate between app.listen() and server.listen() in Express.js.

app.listen() is a convenient method in Express.js that internally uses server.listen() to start the server. The server.listen() method is part of the Node.js core HTTP module and provides more control over the server instance. While app.listen() is easier to use for basic setups, server.listen() is preferred when you need access to the http.Server instance for advanced configurations.

What are some popular alternatives to Express JS?

Some popular alternatives to Express.js include:

• Koa: Developed by the same team as Express, Koa is a lightweight and more modern framework for building APIs.
• Hapi: A rich framework for constructing packages and offerings, with a focus on configuration over code.
• Js: A full-featured MVC framework that mimics the sample of frameworks like Ruby on Rails however built on Node.Js.

Compare Express.js with other Node.js frameworks like Koa, Hapi, and Sails.

How do you enable CORS in an Express.js application?

To enable CORS (Cross-Origin Resource Sharing) in an Express.js application, you can use the Cors middleware. First, install it using npm:

Then, include and configure it in your Express app:

This allows cross-origin requests to your Express.js application from any domain.

Explain what CORS is in Express JS.

CORS, or Cross-Origin Resource Sharing, is a safety function implemented in web browsers to manipulate how resources from one origin can be requested from another foundation. In Express.js, allowing CORS allows your server to address requests from specific domain names, which is important for APIs that serve assets to web applications hosted on unique domains.

What is the .env file used for?

The .env record is used to make surroundings variables in a key-pair format. These variables are then accessed on your software using the .env object. Storing confidential information like API keys, database URLs, and secret tokens in a .env record allows for preserving them out of the supply code, enhancing protection and flexibility.

How do you retrieve the URL parameters from a GET request in Express.js?

You can make use of the req.params object. Here’s an example:

Here, if the URL is /user/123, the user ID is 123, the server responds with “User ID: 123”.

How can you configure a database connection in an Express.js application?

Here, you can get a database connection in your Express.js application and use a database client. For example, you can take Mongoose for MongoDB or pg for PostgreSQL. For example, using Mongoose you can configure a database connection:

This code establishes a connection to the MongoDB database named mydatabase.

Which major tools can be integrated with Express JS?

Express.js can be integrated with several major tools to enhance its functionality:

• Mongoose: An ORM for MongoDB to handle database operations.
• Passport.js: Authentication middleware for securing routes.
• JWT: JSON Web Tokens for secure user authentication.
• Morgan: HTTP request logger middleware.
• Nodemailer: For sending emails from your application.
• Helmet: Security middleware to protect the app from various vulnerabilities.

Compare different template engines supported by Express.js: Pug, EJS, Handlebars, and Mustache.

Explain what HTTP methods are in Express.js.

HTTP methods in Express.js correspond to the different types of requests that clients can make to a server. The most commonly used methods are:

• GET: Retrieve data from the server.
• POST: Send data to the server, often to create or update resources.
• PUT: Replace all current representations of the target resource with the request payload.
• DELETE: Remove a specified resource.
• PATCH: Apply partial modifications to a resource. These methods map to app.get(), app.post(), app.put(), app.delete(), and app.patch() in Express.js.

Compare HTTP Status Codes (200, 400, 401, 403, 404, 500) in Express.js.

How would you configure properties in Express.js?

In Express.js, you can configure application properties using app.set() and app.get(). For example, to set the view engine:

This configures Express.js to use Pug as the templating engine. You can retrieve the configuration with app.get():

How would you configure Portin Express JS?

Configuring ports in Express.js involves using methods like app.set() to customise application settings. For example, setting the port for the server:

This sets the port the application will listen on. Retrieving this property can be done using:

These methods help manage and customise the behaviour of your Express application.

How do you use the express-validator library for input validation?

To use express-validator for input validation, first install the library:

Then, use it in your routes like this:

This code checks that the email is valid and the password is at least 5 characters long.

Create a simple middleware for validating users.

Here’s an example of a simple Express.js middleware for validating users:

This middleware checks if the username and password fields are provided in the request body before allowing access to the route.

What is the role of the express.Router class?

The express.Router class in Express.js is used to create modular, mountable route handlers. It allows you to define routes in a separate module, making your application more organised and scalable. You can also create multiple routers. Here each router handles different parts of your application. Then you can mount them onto the main app using app.use().

What is the difference between Express.js and Node.js?

What is third-party middleware?

Third-party middleware in Express.js refers to middleware functions that aren’t built into Express but are available through outside packages. Examples encompass body-parser for parsing requests in our bodies, cors for enabling go-foundation requests, and morgan for logging HTTP requests. These middleware capabilities can be without problems integrated into an Express utility to extend its capability.

Differentiate between Middleware types: Application-level, Router-level, Error-handling, and Built-in in Express.js.

How can you set up a basic authentication middleware in Express.js?

To install primary authentication middleware in Express.js, you can create a middleware feature that checks the Authorization header for a legitimate username and password. Here’s an easy example:

In this situation, YWRtaW46cGFzc3dvcmQ= is the Base64 encoded string for admin:password. The middleware tests for this encoded string and both let in the request to proceed or send a 401 Unauthorised reaction.

Write a code to create a custom error-handling middleware in Express.js.

Here’s how you can create a custom error-handling middleware in Express.js:

This middleware function captures any errors that occur during request processing. After that, it logs the error stack to the console and sends a generic error message to the client.

How can you implement session management using Express-session?

To implement session management in Express.js using express-session, follow these steps:

Install express-session:

Configure and use the session middleware:

This code sets up session management, where the number of page views is stored in the session.

How do you implement pagination in Express.js?

Pagination in Express.js can be implemented by determining the limit and offset based on query parameters. Here’s a simple example:

This code takes page and limit as query parameters and slices the items array to return only the requested page.

How do you handle file uploads in Express.js using Multer?

To handle file uploads in Express.js, you can use the multer middleware:

Install multer:

This code sets up a document and adds a route in which documents are stored to the uploads/ listing, and the filename is returned to the consumer.

What is Built-in middleware?

Built-in middleware in Express.js are pre-defined middleware capabilities furnished utilising Express to handle unusual responsibilities. Examples include:

• express.json(): Parses incoming JSON requests in req.Body.
• express.urlencoded(): Parses URL-encoded in req.Body.
• express.static(): Serves static documents like images, CSS, and JavaScript from a certain listing.

These middleware functions simplify common tasks without requiring external dependencies.

How do you handle errors in Express.js?

In Express.js, errors can be handled with the use of custom errors-handling middleware. This middleware captures errors and determines an appropriate reaction. Here’s an example:

This middleware logs the error stack and sends a 500 status code with a generic error message to the client.

Compare error-handling techniques in Express.js.

Write a code to handle 404 errors in Express.js.

To handle 404 errors, you can add a middleware function that catches all unmatched routes:

Explain the use of next() in Express.js middleware.

The next() function in Express.js is used within middleware functions to pass control to the next middleware or route handler. Without calling next(), the request will be stopped, and no further processing will occur. It’s essential for chaining multiple middleware functions together to handle requests effectively.

Compare synchronous and asynchronous middleware in Express.js.

Explain the structure of an Express JS application.

An Express.js application generally has a well-defined structure to manage different parts of the application efficiently. The structure typically includes:

• app.js: The main entry point of the application where the Express app is configured and routes are set up.
• routes/: A directory containing route definitions and handlers, each in separate modules.
• views/: A directory for view templates if using a templating engine like Pug or EJS.
• public/: A directory for serving static assets like images, CSS, and JavaScript files.
• middlewares/: A directory to store custom middleware functions.
• config/: Configuration files such as database settings and environment variables.

How can you create a RESTful API using Express.js?

In this, you can define different routes that correspond to different HTTP methods and perform CRUD operations. For example: 

This code sets up a basic RESTful API with routes for getting, creating, updating, and deleting items.

What is the difference between app.route() and app.use() in Express.js?

• app.route(): In this method, you can define multiple route handlers for a specific route. You can also chain them for different HTTP methods like GET, POST, etc. Hence, it helps in keeping clean and organised code for a single route. 
• app.use(): This method is used to mount middleware or routes at a particular path. You can apply the middleware globally across all routes or across any specific path. This is also more general-purpose and handles a lot of middleware functions. 

Differentiate between app.use() and app.route() in Express.js.

Which are the arguments available to an Express JS route handler function?

An Express.js route handler function can have the following arguments:

• req (Request): Represents the HTTP request and contains data like query parameters, body, headers, etc.
• res (Response): Represents the HTTP response that will be sent to the client.
• next (Next): A function that passes control to the next middleware function in the stack.
• err (Error): When used in error-handling middleware, it represents an error object.

What is JWT?

JWT (JSON Web Tokens) includes 3 components: Header, Payload, and Signature. JWTs are normally used for authentication, where the server generates a token upon a successful login, and the customer uses this token to get the right of entry to covered routes inclusive of it in the Authorization header.

Write a code to send emails in an Express.js application using Nodemailer.

To send emails using Nodemailer in an Express.js application:

Install Nodemailer:

Set up and use Nodemailer in your route:

This code sets up an Express route that sends an email using Gmail’s SMTP server when accessed.

What is the purpose of the app.locals object in Express.js?

The app.locals item in Express.js gives a way to use global variables which might be available to the application’s views. Variables are saved in app.locals may be accessed in any view rendered by using the app, making it beneficial for sharing commonplace statistics like website-huge settings, titles, or application functions across extraordinary perspectives.

What is router-level middleware?

Router-level middleware is middleware that is bound to an instance of express.Router().t works similarly to application-level middleware but is scoped to the specific router instance, allowing you to apply middleware only to certain parts of your application.

Differentiate between app.locals and res.locals in Express.js.

How do you implement a file download feature using Express.js?

To implement a file download feature in Express.js, you can use the res.download() method:

This code allows clients to download the sample.pdf file when they visit the /download route.

How can you protect routes in Express.js using JWT?

Here, you can create middleware that verifies the JWT token before granting access:

What is meant by Scaffolding in Express JS?

Scaffolding in Express.js refers to the technique of generating a basic structure in your software, inclusive of directories and boilerplate code. This helps developers quickly install a brand-new project with a constant structure, saving time on repetitive tasks. Express gives a command-line tool for scaffolding packages, which generates documents and directories based on quality practices.

Do Other MVC frameworks also support scaffolding?

Yes, many MVC frameworks support scaffolding. For example:

• Ruby on Rails: IT provides you a robust scaffolding so that you can generate different models, views and controllers with a single command.
• ASP.NET: It also offers scaffolding tools for generating code of controllers and views.
• Laravel: This is a PHP framework that supports scaffolding so you can create basic CRUD operations (Create, Read, Update, Delete). 

How would you install an Express application generator for scaffolding?

To install the Express application generator, which is used for scaffolding, run the following command globally:

npm install -g express-generator

What is Yeoman and how to install Yeoman for scaffolding?

Yeoman is a tool that enables developers to generate scaffolding for net packages. It makes use of ‘generators’ to create an undertaking structure, including documents and directories, primarily based on fine practices.

To install Yeoman:

After installing Yeoman, you can use it with various generators to scaffold projects for different frameworks and languages.

Why should you separate the Express app and server?

Separating the Express app and server is a best practice that improves code modularity and maintainability. By splitting the app logic (routes, middleware, configurations) from the server setup (port listening), you can easily test the application logic independently, reuse the app instance in different environments, and make the codebase more organised.

Compare Body Parsers: body-parser, express.json(), express.urlencoded() in Express.js.

What do you understand about ESLint?

ESLint is a broadly used JavaScript linter that analyses your code to discover and fix troubles primarily based on configurable rules. It helps keep code up to high quality and consistent by catching syntax errors, enforcing coding requirements, and suggesting improvements. Developers can personalise ESLint by way of defining specific regulations and integrating them into their development workflow through IDE plugins or command-line tools.

Compare Express.js methods: res.send(), res.json(), and res.render().

What is Bcrypt used for?

Bcrypt is a password-hashing library used to safely save passwords. It affords a one-manner hash function that makes it computationally difficult to reverse the hash back to the unique password. Bcrypt also consists of a salting mechanism that adds random statistics to the password before hashing, ensuring that even if two customers have an equal password, their hashed values will be one of a kind. This complements safety towards attacks like rainbow tables and brute-pressure attacks.

How can you enable debugging in an Express.js app?

To enable debugging in an Express.js app, you can use the built-in DEBUG environment variable. Set this variable to express:* to enable debugging for all Express.js modules. For example, in your terminal, you can run:

This will output detailed logs related to Express.js operations, such as routing and middleware execution, helping you diagnose issues during development.

Mention some of the databases with which Express JS is integrated.

Express.js can be integrated with various databases, both SQL and NoSQL, depending on the application’s needs. Some commonly used databases include:

• MongoDB: A NoSQL database often used with Mongoose ORM in Express applications.
• PostgreSQL: Another SQL database known for its robustness and used with libraries like pg or sequelize.
• SQLite: This is a lightweight SQL database that you can use in smaller applications or for development purposes.
• Redis: An in-memory data store used for caching and session management in Express applications.

What is the difference between Express and Django?

How do you deal with error handling in Express.js? Explain with an example.

Error handling in Express.js is typically done using custom error-handling middleware. Here’s an example:

This middleware captures any errors that arise at some stage in the request processing, logs the error stack, and sends a 500 reputation code with a widespread error message to the consumer. By setting this middleware in any case routes, it ensures that any unhandled errors are caught and controlled gracefully.

Explain the concept of the test pyramid.

The test pyramid is an idea that emphasises the importance of different sorts of trying out in software program development. It shows that maximum assessments must be unit exams (at the bottom of the pyramid), which are rapid and dependable. Fewer tests should be integration tests (in the middle), which test the interaction between different modules. The least number of tests should be end-to-end tests (at the top), which test the entire application but are slower and more fragile. This approach helps ensure comprehensive testing coverage while maintaining efficiency.

Compare different ways to secure Express.js applications: Helmet, Rate Limiting, CORS.

Which template engines do Express support?

Express.js supports several template engines for rendering dynamic HTML content. Some of the popular ones include:

• Pug (formerly Jade): Known for its minimalistic and indentation-based syntax.
• EJS (Embedded JavaScript): Allows embedding JavaScript into HTML, similar to other templating engines like PHP or JSP.
• Handlebars: Extends the basic Mustache templating engine with features like helpers and partials.
• Mustache: A logic-less templating engine that focuses on simplicity and fast rendering.

How do you serve static files using Express.js?

To serve static files in Express.js, such as images, CSS, and JavaScript files, you can use express.static middleware. Here’s an example:

This code serves all files located in the public directory at the root URL. For instance, a file named style.css in the public directory would be accessible at http://yourdomain.com/style.css.

How do you retrieve the URL parameters from a GET request in Express.js?

To retrieve URL parameters in Express.js, use the req.params object. Here’s an example:

In this example, if the URL is /user/123, the userId will be 123, and the server will respond with “User ID: 123”. This is useful for dynamic routes where the parameters in the URL dictate the response.

Conclusion

Express.js is an effective and flexible framework that simplifies web application improvement. Whether you’re a novice or an experienced developer, gaining knowledge of the key principles and practices in Express.js can appreciably boost your potential to construct efficient and scalable server-side programs. The interview questions shown in this guide provide a strong basis to put you together for technical discussions and actual-world demanding situations.

By knowing the core functions, best practices, and common use cases of Express.js, you may confidently tackle all the types of interview questions that are asked to you related to Express.js and demonstrate your skills. Keep practising and exploring, as Express.js remains an important device in modern web development with frequent updates.