**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** Routing and Middleware **Topic:** Error handling middleware and logging requests In the previous topics, we covered the basics of routing and middleware in Express.js. Now, let's dive deeper into two crucial aspects of building robust web applications: error handling middleware and logging requests. ### Error Handling Middleware Error handling middleware functions are designed to catch and handle errors that occur during the execution of a request. These functions are typically placed at the end of the middleware chain, and they should be designed to handle any errors that may have occurred during the processing of a request. In Express.js, you can define error-handling middleware functions using the following syntax: ```javascript app.use((err, req, res, next) => { // error handling code here }); ``` The `err` parameter represents the error that occurred, while `req`, `res`, and `next` are the same parameters that we're familiar with in regular middleware functions. Here's an example of a simple error handling middleware function: ```javascript app.use((err, req, res, next) => { console.error(err); res.status(500).send('Internal Server Error'); }); ``` In this example, the middleware function logs the error to the console and returns a 500 Internal Server Error response to the client. ### Logging Requests Logging requests is an essential aspect of building robust web applications. It allows you to monitor and debug your application's behavior, and it can also help you identify potential security vulnerabilities. In Express.js, you can log requests using the `morgan` middleware. `morgan` is a popular logging middleware that can be used to log HTTP requests. First, install `morgan` using npm: ```bash npm install morgan ``` Then, add the following code to your Express.js application: ```javascript const morgan = require('morgan'); app.use(morgan('dev')); ``` The `'dev'` parameter specifies the logging format. You can customize the logging format by passing a custom string to the `morgan()` function. Here are some common logging formats: * `combined`: The Apache common log format * `common`: The Apache common log format without the hostname * `dev`: A concise format for development and testing * `short`: A shorter format for development and testing * `tiny`: A very concise format for development and testing You can also customize the logging format by passing a custom string to the `morgan()` function. For example: ```javascript app.use(morgan(':method :url :status :response-time ms')); ``` This will log the HTTP method, URL, status code, and response time for each request. ### Best Practices for Error Handling Middleware Here are some best practices for error handling middleware: * **Catch all errors**: Make sure to catch all errors that may occur during the execution of a request. This includes errors that may occur in your application code, as well as errors that may occur in third-party libraries. * **Log errors**: Log all errors that occur during the execution of a request. This will help you identify and debug issues with your application. * **Return a meaningful response**: Return a meaningful response to the client, such as a 500 Internal Server Error response. * **Avoid crashing the server**: Avoid crashing the server when an error occurs. Instead, return a meaningful response to the client and log the error for further analysis. ### Best Practices for Logging Requests Here are some best practices for logging requests: * **Log all requests**: Log all requests that come into your application, including successful requests and requests that result in errors. * **Log request metadata**: Log request metadata, such as the HTTP method, URL, and query string. * **Log response metadata**: Log response metadata, such as the status code and response time. * **Use a standardized logging format**: Use a standardized logging format, such as the Apache common log format, to make it easier to analyze and process logs. ### Conclusion Error handling middleware and logging requests are two crucial aspects of building robust web applications. By following best practices for error handling middleware and logging requests, you can ensure that your application is reliable, secure, and performant. In the next topic, we'll cover creating custom middleware functions. If you have any questions or need further clarification on any of the concepts covered in this topic, please [leave a comment below](#comments). **External Resources:** * [Express.js Error Handling Middleware Documentation](https://expressjs.com/en/guide/error-handling.html) * [Morgan Middleware Documentation](https://github.com/expressjs/morgan) * [Logging Best Practices for Node.js Applications](https://www.loggly.com/blog/logging-best-practices-for-node-js/) **Additional Reading:** * [Error Handling in Node.js](https://www.digitalocean.com/community/tutorials/nodejs-error-handling) * [Logging in Node.js](https://www.nodejs.org/en/docs/guides/logging/) We'll cover creating custom middleware functions in the next topic.