Express Error Handling – Middleware for Production and Development

Error handling is the process of detecting and responding to errors that occur during the execution of a program or application. In the context of web applications, error handling typically involves detecting errors that occur during the handling of HTTP requests and responding appropriately to those errors. Effective error handling is important because it can help improve the reliability and availability of an application. 

Express provides middleware functions that can be used to handle errors that occur during the handling of HTTP requests. These middleware functions are added to the application’s middleware stack and are executed when an error occurs during the processing of a request.

Syntax:

function errorHandler(err, req, res, next) {
     // error handling code goes here
}

Error handling in production: Setting up the project.

Step 1: Create a directory for our project and make that our working directory.

$ mkdir gfg
$ cd gfg 

Step 2: Use the npm init command to create a package.json file for our project.

npm init -y

Step 3: Now install Express.

npm i express

In this example, we’ve defined an error-handling middleware function that logs the error to the console and sends a generic error message to the client with a 500 Internal Server Error status code. We’ve also defined a route handler for the root URL that simply sends a “Hello World!” message to the client. To test the error handling middleware, we have intentionally caused an error by adding a route handler that throws an error.

Javascript

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

// Production error handling middleware
app.use(function (err, req, res, next) {
    console.error(err);
    res.status(500).send("Internal Server Error");
});

// Route handlers
app.get("/", function (req, res) {
    res.send("Hello World!");
});
app.get("/error", function (req, res) {
    throw new Error("Something went wrong!");
});

// Start the server
const port = process.env.PORT || 3000;
app.listen(port, function () {
    console.log(`Server listening on port ${port}`);
});

Steps to run the code:

Step 1: Run the following code in the terminal:

node index.js

Step 2: Now create a get request to “localhost:3000/error” to get the error. You can use Postman or write the URL in any web browser.

Output:

Error handling in production: In this example, we’ve defined an error handling middleware function that logs the error stack trace to the console and sends the stack trace to the client with a 500 Internal Server Error status code. We’ve also added a check to ensure that this error-handling middleware is only used in development mode.  To test the error handling middleware, we have intentionally caused an error by adding a route handler that throws an error.

Javascript

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

// Development error handling middleware
if (app.get("env") === "development") {
    app.use(function (err, req, res, next) {
        console.error(err.stack);
        res.status(500).send(`<pre>${err.stack}</pre>`);
    });
}

// Route handlers
app.get("/", function (req, res) {
    res.send("Hello World!");
});
app.get("/error", function (req, res) {
    throw new Error("Something went wrong!");
});

// Start the server
const port = process.env.PORT || 3000;
app.listen(port, function () {
    console.log(`Server listening on port ${port}`);
});

Steps to run the code:

Step 1: Run the following code in the terminal:

node index.js

Step 2: Now create a get request to “localhost:3000/error” to get the error. You can use Postman or write the URL in any web browser.

Output:

Difference between Express error handling middleware for production and development:

	Production	Development
Error response	Generic error message	Detailed error message with stack trace
Error logging	Limited logging, e.g. console	Extensive logging, e.g. console, file, or service
Error debugging	Limited debugging	Extensive debugging with stack trace
Error display	User-friendly error page	Technical error message or stack trace
Error information	Limited information	Detailed information, e.g. request details
Error handling time	Minimise downtime	Maximise debugging time
Middleware usage	Standard middleware	Conditional middleware based on NODE_ENV