グローバルエラー処理の戦略
一元化されたエラー処理の仕組みを実装し、例外を適切に処理して一貫したエラーレスポンスを返します。
「グローバルエラー処理の戦略」はCoddyKit上の無料Node.js Backend Development Bootcampレッスンです。 これはレッスン2/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはNode.js Backend Development Bootcamp学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Node.js Backend Development Bootcampコースには全4レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
Centralized Errors in Express
When building an API, errors are inevitable. How you handle them can drastically affect user experience and application stability.
Scattered try...catch blocks in every route can become messy and inconsistent. This lesson introduces global error handling in Express.js.
Express's Default Error Response
By default, if an error occurs in an Express route or middleware and isn't caught, Express will:
- Send a response with a
500 Internal Server Errorstatus code. - Include the error's stack trace in the response.
This default behavior is not ideal for production, as it exposes sensitive server details to clients.
The Special Error Middleware
Express recognizes a special type of middleware for error handling. Unlike regular middleware which takes (req, res, next), error-handling middleware takes four arguments:
err: The error object passed by Express.req: The request object.res: The response object.next: The next middleware function.
Express knows to skip all other middleware and send the error directly to this special handler.
Your First Error Middleware
Let's create a basic error handling middleware. This middleware should be placed after all other routes and middleware in your app.js file.
Try running this simple Express app:
const express = require('express');
const app = express();
// A route that intentionally throws an error
app.get('/error', (req, res, next) => {
const err = new Error('Something went wrong!');
err.statusCode = 400;
next(err); // Pass the error to the error middleware
});
// Global Error Handling Middleware (must be last)
app.use((err, req, res, next) => {
console.error(err.stack); // Log the error for debugging
const statusCode = err.statusCode || 500;
res.status(statusCode).json({
status: 'error',
message: err.message || 'An unexpected error occurred!'
});
});
const PORT = 3000;
app.listen(PORT, () => {
console.log(`Server running on port ${PORT}`);
console.log('Visit http://localhost:3000/error to see error handling');
});Operational vs. Programmatic Errors
It's crucial to distinguish between error types:
- Operational Errors: Predictable errors from system operations (e.g., invalid user input, network issues, resource not found). These are 'soft' errors we can handle gracefully and send specific messages to the client.
- Programmatic Errors: Bugs in your code (e.g., trying to read property of
undefined, database connection failures). These are 'hard' errors that indicate a problem with your application logic and might require restarting the process.
Our global handler should treat these differently.
Crafting Custom Errors
To better categorize and handle errors, we can create custom error classes. This allows us to attach specific properties like statusCode and isOperational to our errors.
Here's a simple AppError class:
// utils/appError.js
class AppError extends Error {
constructor(message, statusCode) {
super(message);
this.statusCode = statusCode;
this.status = `${statusCode}`.startsWith('4') ? 'fail' : 'error';
this.isOperational = true;
Error.captureStackTrace(this, this.constructor);
}
}
module.exports = AppError;Using Custom Errors in Routes
Now that we have our AppError class, we can use it in our routes to throw specific, operational errors. When an AppError is thrown and caught by next(err), our global handler can provide a tailored response.
Run this example and try visiting /user/123 (valid) and /user/abc (invalid ID):
const express = require('express');
const AppError = require('./utils/appError'); // Assuming appError.js is in a 'utils' folder
const app = express();
// Example route using custom error
app.get('/user/:id', (req, res, next) => {
const userId = parseInt(req.params.id);
if (isNaN(userId)) {
return next(new AppError('Invalid user ID provided!', 400));
}
res.status(200).json({
status: 'success',
data: { id: userId, name: `User ${userId}` }
});
});
// Global Error Handling Middleware (must be last)
app.use((err, req, res, next) => {
console.error(err.stack);
const statusCode = err.statusCode || 500;
const status = err.status || 'error';
if (err.isOperational) {
res.status(statusCode).json({
status: status,
message: err.message
});
} else {
// For programmatic errors, send a generic message in production
res.status(500).json({
status: 'error',
message: 'Something went very wrong!'
});
}
});
const PORT = 3000;
app.listen(PORT, () => {
console.log(`Server running on port ${PORT}`);
console.log('Visit http://localhost:3000/user/1 to see success');
console.log('Visit http://localhost:3000/user/abc to see custom error');
});
// --- Create utils/appError.js with the content from the previous scene for this to run ---Intelligent Error Responses
Our error handler can be further refined to provide different responses based on the environment (development vs. production).
- Development: Send full error details (stack trace) for debugging.
- Production: Send minimal, user-friendly messages for operational errors and generic messages for programmatic errors (to hide internal details).
This approach keeps your API secure and informative.
Catching the Unforeseen
Express middleware only catches errors that occur within the request-response cycle and are passed with next(err). What about errors outside of this?
- Uncaught Exceptions: Synchronous errors that are not handled by any
try...catchblock. - Unhandled Rejections: Promise rejections that don't have a
.catch()handler.
Node.js provides global process event listeners for these critical errors, which should ideally shut down the application after logging the error.
// In your server.js or app.js, before app.listen
process.on('uncaughtException', err => {
console.error('UNCAUGHT EXCEPTION! Shutting down...');
console.error(err.name, err.message, err.stack);
process.exit(1); // Exit with failure code
});
// After app.listen, for unhandled promise rejections
process.on('unhandledRejection', err => {
console.error('UNHANDLED REJECTION! Shutting down...');
console.error(err.name, err.message);
// Optionally close server first, then exit
server.close(() => {
process.exit(1);
});
});Error Handling Challenge
You've learned about Express error handling. Which of the following is the correct signature for an Express error handling middleware?
Global Error Handling Summary
Great job! You've learned how to implement robust error handling in your Express applications.
- Express uses a special 4-argument middleware for error handling.
- Distinguish between operational and programmatic errors.
- Create custom error classes to add context to your errors.
- Implement global listeners for uncaught exceptions and unhandled rejections.
Centralized error handling makes your API more reliable, secure, and easier to debug!
よくある質問
「グローバルエラー処理の戦略」レッスンは無料ですか？
はい。「グローバルエラー処理の戦略」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Node.js Backend Development Bootcampコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Node.js Backend Development Bootcampコースには全4レッスンが含まれています。
「グローバルエラー処理の戦略」で何を学びますか？
一元化されたエラー処理の仕組みを実装し、例外を適切に処理して一貫したエラーレスポンスを返します。 ブラウザで直接実行するハンズオンコードでNode.js Backend Development Bootcampを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
Node.js Backend Development Bootcampを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのNode.js Backend Development Bootcampは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン2/4です。
「グローバルエラー処理の戦略」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このNode.js Backend Development Bootcampレッスンでコードを書いて実行できますか？
はい。すべてのNode.js Backend Development Bootcampレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。