Tutorial: Error Handling and Middleware in Express.js

1. Introduction

In this tutorial, we aim to explore error handling and middleware in Express.js. Express.js is a fast, unopinionated, and minimalist web application framework for Node.js.

By the end of this tutorial, you will:

• Understand how middleware works in Express.js
• Learn how to handle errors in Express.js
• Be able to develop middleware functions and use them for error handling

The prerequisites for this tutorial are a basic understanding of JavaScript and some familiarity with Node.js and Express.js.

2. Step-by-Step Guide

Understanding Middleware

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. The next middleware function is commonly denoted by a variable named next.

Middleware functions can perform the following tasks:

• Execute any code
• Make changes to the request and the response objects
• End the request-response cycle
• Call the next middleware function in the stack

Error Handling in Express

In Express, 404 responses are not the result of an error, so the error-handler middleware will not capture them. To handle a 404 response, add a middleware function at the end of the stack.

Errors that occur in synchronous code inside route handlers and middleware require no extra work. If synchronous code throws an error, then Express will catch and process it.

3. Code Examples

Basic Error Handling

app.use(function(err, req, res, next) {
  console.error(err.stack); // Log error message in our server's console
  res.status(500); // Announce that this is a 500 error
  res.send('Something broke!'); // Inform client about the error
});

In this example, app.use is used to setup middleware. We're passing a function that accepts four arguments to app.use. This function is an error handling middleware. Express identifies it as such and passes errors to it.

404 Handling

app.use(function(req, res, next) {
  res.status(404).send('Sorry cant find that!');
});

This middleware function does not have an error in its arguments, Express treats it as a regular middleware. You should ensure this comes after all your other routes because responses are sent in order.

4. Summary

In this tutorial, we have learned about error handling and middleware in Express.js. We've seen how Express uses middleware and how to create error handling middleware.

For further learning, consider exploring more about Express middleware, like built-in middleware, third-party middleware, and writing your own middleware functions.

5. Practice Exercises

1. Create a middleware function that logs the current date and time each time a request is made to the server.
2. Create an error handling middleware that responds with a custom error page.
3. Create a middleware stack with at least three middleware functions, where each function logs something to the console and passes control to the next middleware function.

Tips: Always handle errors and be prepared for the unexpected. Remember, middleware functions that do not handle errors must take only three parameters. Otherwise, they might be mistaken for error handling functions.