Error handling in Express.js
Error handling is a critical aspect of developing robust and reliable web applications. In Express.js, handling errors effectively ensures that your application can gracefully recover from unexpected issues and provide meaningful feedback to users. This chapter covers error handling in Express.js, from basic concepts to advanced techniques. We will explore different types of errors, how to catch and handle them, and best practices for creating a resilient application.
Bonus💡: Practical example at the end of article
Error Handling
Error handling involves detecting, managing, and responding to errors that occur during the execution of your application. Effective error handling ensures your application remains stable and provides useful feedback to users and developers.
Types of Errors in Express.js
There are several types of errors that can occur in an Express.js application:
- Syntax Errors: Mistakes in the code syntax.
- Runtime Errors: Errors that occur during the execution of the application.
- Operational Errors: Errors related to system operations, such as network failures or file not found errors.
- Logical Errors: Errors in the application logic, resulting in incorrect behavior.
Basic Error Handling
Using Try-Catch Blocks
The try-catch statement allows you to handle errors in synchronous code.
Example:
const express = require('express');
const app = express();
const port = 3000;
app.get('/', (req, res) => {
try {
// Simulate an error
throw new Error('Something went wrong!');
} catch (err) {
res.status(500).send('Internal Server Error');
}
});
app.listen(port, () => {
console.log(`Server is running on http://localhost:${port}`);
});
Output:
Visiting http://localhost:3000 will display “Internal Server Error” due to the simulated error.
Express.js Default Error Handler
Express.js has a built-in error handler that catches and handles errors thrown in route handlers and middleware.
Example:
app.get('/', (req, res) => {
throw new Error('Something went wrong!');
});
Output:
Express.js will catch the error and respond with a 500 Internal Server Error.
Custom Error Handlers
Creating Custom Error Handlers
Custom error handlers allow you to define specific behavior for different types of errors.
Example:
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
Handling 404 Errors
A common requirement is to handle 404 Not Found errors.
Example:
app.use((req, res, next) => {
res.status(404).send('Sorry, we could not find that!');
});
Error Handling Middleware
Defining Error Handling Middleware
Error handling middleware is a special type of middleware with four arguments: err, req, res, and next.
Example:
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
Using Next() Function
The next() function is used to pass control to the next middleware or error handler.
Example:
app.get('/', (req, res, next) => {
const err = new Error('Something went wrong!');
next(err);
});
Logging Errors
The simplest way to log errors is to use console.error.
Example:
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
Integrating with Logging Libraries
For more robust logging, integrate with logging libraries like Winston or Morgan.
Example with Winston:
const winston = require('winston');
const expressWinston = require('express-winston');
app.use(expressWinston.errorLogger({
transports: [
new winston.transports.Console()
]
}));
Advanced Error Handling
Handling Asynchronous Errors
Use try-catch with async/await to handle errors in asynchronous code.
Example:
app.get('/', async (req, res, next) => {
try {
await someAsyncFunction();
res.send('Hello, world!');
} catch (err) {
next(err);
}
});
Handling Errors in Promises
Handle errors in promises using .catch.
Example:
app.get('/', (req, res, next) => {
somePromiseFunction()
.then(result => {
res.send(result);
})
.catch(err => {
next(err);
});
});
Centralized Error Handling
Centralized error handling ensures that all errors are processed in a single place.
Example:
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).send('Something broke!');
});
Best Practices for Error Handling
- Use Centralized Error Handling: Centralize your error handling to avoid duplicating error handling logic.
- Graceful Shutdown: Ensure your application can shut down gracefully in case of critical errors.
- Log Errors: Log errors for debugging and monitoring purposes.
- Return Meaningful Responses: Provide meaningful error messages to users, but avoid exposing sensitive information.
- Use Monitoring Tools: Integrate with monitoring tools to detect and alert on errors in production.
Practical example
let’s build a full practical example demonstrating error handling in an Express.js application. This example will include handling different types of errors, logging, and using custom error handlers.
Project Structure
error-handling-example/
├── views/
│ └── error.ejs
├── index.js
├── package.json
└── app.js
Setting Up the Project
1.Initialize the project:
mkdir error-handling-example
cd error-handling-example
npm init -y
npm install express ejs winston
2.Create the index.js file:
const express = require('express');
const app = require('./app');
const port = 3000;
app.listen(port, () => {
console.log(`Server is running on http://localhost:${port}`);
});
3.Create the app.js file:
const express = require('express');
const winston = require('winston');
const app = express();
// Setting up the view engine
app.set('view engine', 'ejs');
app.set('views', './views');
// Middleware to simulate a route with an error
app.get('/error', (req, res, next) => {
const err = new Error('Simulated error');
err.status = 500;
next(err);
});
// Middleware to simulate a 404 Not Found error
app.use((req, res, next) => {
const err = new Error('Not Found');
err.status = 404;
next(err);
});
// Error handling middleware
app.use((err, req, res, next) => {
// Log the error using winston
winston.error(`${err.status || 500} - ${err.message} - ${req.originalUrl} - ${req.method} - ${req.ip}`);
// Respond with a user-friendly message
res.status(err.status || 500);
res.render('error', {
message: err.message,
error: process.env.NODE_ENV === 'development' ? err : {}
});
});
module.exports = app;
4.Create the views/error.ejs file:
<!DOCTYPE html>
<html>
<head>
<title>Error</title>
</head>
<body>
<h1>Error</h1>
<h2><%= message %></h2>
<% if (error.status) { %>
<h3>Status: <%= error.status %></h3>
<% } %>
<% if (error.stack) { %>
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Explanation
Project Initialization:
We create a new Node.js project and install the necessary dependencies (express, ejs, and winston).
Server Entry Point (index.js):
- This file sets up the Express.js server and listens on port 3000.
Application Configuration (app.js):
- View Engine Setup: Configure Express to use EJS as the templating engine.
- Error Simulation Route: A route
/errorthat throws a simulated error. - 404 Error Middleware: Middleware to handle 404 Not Found errors.
- Error Handling Middleware: Middleware that handles all errors. It logs errors using Winston and responds with an error page.
Error View Template (views/error.ejs):
- An EJS template that displays error information to the user.
Start the server:
node index.js
Visit the following URLs in your browser:
- http://localhost:3000/error to see the simulated error.
- http://localhost:3000/nonexistent to see the 404 Not Found error.
Output for /error:
<!DOCTYPE html>
<html>
<head>
<title>Error</title>
</head>
<body>
<h1>Error</h1>
<h2>Simulated error</h2>
<h3>Status: 500</h3>
<pre>Error: Simulated error
at /path/to/your/project/app.js:10:12
...
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Error handling is a crucial aspect of developing reliable and maintainable Express.js applications. By understanding the different types of errors, using custom error handlers, and following best practices, you can create robust applications that handle errors gracefully. Remember to log errors and use monitoring tools to keep track of issues in your application. Happy coding !❤️