Creating Custom Middleware in Node.js

1. Introduction

In this tutorial, we'll learn how to create custom middleware in Node.js. 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 execute any code, make changes to the request and response objects, end the request-response cycle, and call the next middleware function in the stack.

By the end of this tutorial, you will be able to:

• Understand what middleware is in Node.js
• Write your own custom middleware functions
• Utilize your own middleware within your routes

Prerequisites

To follow along with this tutorial, you should have:

• A basic understanding of JavaScript
• Node.js and npm (Node Package Manager) installed on your machine
• Basic knowledge of Express.js, a popular Node.js framework

2. Step-by-Step Guide

Middleware functions are crucial in Express applications. They can be used for a variety of things, including handling requests, changing the request or response, and ending the request-response cycle.

Writing a Middleware Function

A middleware function in Express is a function that has access to the request object, the response object, and the next middleware function in the application's request-response cycle.

Here's a template for creating a middleware function:

function myMiddleware(req, res, next) {
  // middleware function code
  // next is a function that, when invoked, executes the next middleware function
  next();
}

In the above example, myMiddleware is a middleware function. It has access to the request object (req), the response object (res), and the next function.

Using the Middleware Function

To use our middleware function, we call app.use() and pass in our middleware function:

app.use(myMiddleware);

Now, every time a request is made to our app, myMiddleware will be invoked.

3. Code Examples

Let's create a middleware function that logs the current date and time whenever a request is made.

function logger(req, res, next) {
  console.log(new Date(), req.method, req.url);
  next();
}

app.use(logger);

In the above code:

• We define a middleware function logger that logs the current date and time, the request method, and the request URL.
• We then use app.use to use this middleware in our application. Now, every time a request is made to our app, this function will be invoked and the current date, request method, and request URL will be logged to the console.

4. Summary

In this tutorial, we've learned about middleware in Express.js and how to create and use our own custom middleware functions. We've seen how middleware functions have access to the request and response objects, as well as the ability to execute the next middleware function in the stack.

For further learning, you might explore more about error-handling middleware, built-in middleware, and third-party middleware in Express.js. You can also try creating more complex middleware functions for your Express applications.

5. Practice Exercises

1. Create a middleware function that logs the request headers.

2. Create a middleware function that checks if a user object exists in req.body. If it exists, let the request proceed. Otherwise, send a response with the status code 400 and a message "Bad Request".

3. Create a middleware function that measures the time it takes for a request to travel through the middleware function and logs it.

Remember, the key to mastering middleware (or any concept in programming) is practice. Try to incorporate middleware functions in your own Express.js projects. Happy coding!