Understanding the Basics of Express.js

Creating an Express.js Application

When building a web application using Node.js, one of the most popular frameworks to use is Express.js. It provides a flexible and modular way to handle HTTP requests and responses. In this section, we will explore the basics of Express.js and understand the purpose of the express() function.

The express() Function

The express() function is the main entry point for creating an Express.js application. It returns a new Express application instance, which can be used to configure and start the server.

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

In the above code, we first import the Express.js module using the require() function. Then, we call the express() function to create a new Express application instance, which is stored in the app variable.

Purpose of the express() Function

The express() function serves several purposes:

• It creates a new Express application instance, which can be used to configure and start the server.

• It returns an instance of the Express class, which provides methods for configuring the application, such as setting up middleware, defining routes, and starting the server.

• It allows you to create multiple Express applications, each with its own configuration and settings.

Example Usage

Here is an example of using the express() function to create a simple Express.js application:

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

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

app.listen(3000, () => {
  console.log('Server started on port 3000');
});

In this example, we create a new Express application instance using the express() function. We then define a route for the root URL ('/') using the app.get() method, which sends a response with the text "Hello World!". Finally, we start the server using the app.listen() method, which listens for incoming requests on port 3000.

Conclusion

In conclusion, the express() function is the main entry point for creating an Express.js application. It returns a new Express application instance, which can be used to configure and start the server. Understanding the purpose and usage of the express() function is essential for building robust and scalable web applications using Express.js.

Frequently Asked Questions

Q: What is the purpose of the express() function in Express.js?

A: The express() function creates a new Express application instance, which can be used to configure and start the server.

Q: How do I create a new Express application instance?

A: You can create a new Express application instance by calling the express() function, like this: const app = express();

Q: What is the difference between the express() function and the Express class?

A: The express() function returns an instance of the Express class, which provides methods for configuring the application.

Q: Can I create multiple Express applications?

A: Yes, you can create multiple Express applications by calling the express() function multiple times.

Q: How do I start the Express server?

A: You can start the Express server using the app.listen() method, which listens for incoming requests on a specified port.