Debugging in Express JS

Debugging is an essential part of software development, and Express js is no exception. When building web applications with Express.js, it is common to encounter bugs and errors that need to be fixed. In this article, we will explore the best practices for debugging Express js applications.

Logging in Express JS

Logging is one of the most important tools for debugging Express.js applications. It allows you to see what is happening inside your application, and it can help you identify bugs and errors quickly. Express.js provides a built-in logging middleware called morgan, which logs HTTP requests and responses. You can use morgan to log requests and responses to the console, a file, or any other destination of your choice.

To use morgan, you need to install it using npm:

npm install morgan

Once you have installed morgan, you can use it as middleware in your Express.js application:

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

app.use(morgan('dev'));

In this example, we have used morgan as middleware in our Express.js application. The ‘dev’ option tells morgan to log requests and responses to the console in a developer-friendly format.

Debugging middleware

Express.js allows you to create custom middleware that can be used for debugging purposes. Middleware functions are functions that take three arguments: req, res, and next. They can be used to modify the request and response objects, or they can be used to perform other tasks.

To create a debugging middleware, you can define a function that logs information to the console:

function debugMiddleware(req, res, next) {
  console.log('Request received:', req.method, req.url);
  next();
}

app.use(debugMiddleware);

In this example, we have created a debugging middleware that logs information about the incoming request to the console.

Debugging with breakpoints

Another useful tool for debugging Express.js applications is breakpoints. Breakpoints allow you to pause the execution of your code at a specific point and inspect the values of variables and objects. You can set breakpoints in your code using a debugger, such as the one built into the Chrome Developer Tools.

To use breakpoints with Express.js, you need to start your application in debug mode:

node --inspect index.js

In this example, we have started our Express.js application in debug mode using the –inspect flag. Once your application is running in debug mode, you can open the Chrome Developer Tools and navigate to the “Sources” tab. From there, you can select your application’s source file and set breakpoints by clicking on the line numbers.

Error handling middleware

Error handling is an essential part of debugging Express.js applications. When an error occurs in your application, you need to handle it gracefully and provide feedback to the user. Express.js provides a built-in error handling middleware that can be used to handle errors in your application.

To use the error handling middleware, you need to define a middleware function with four arguments: err, req, res, and next:

function errorHandler(err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Something broke!');
}

app.use(errorHandler);

Output