README.md

@csnw/express-async-errors

This module patches Express to better support errors thrown from async request handlers. The primary difference of this package from express-async-errors is that you are expected to call the patch function explicitly, rather than it applying as soon as the module is require'd or import'ed. Both ESM and CommonJS projects are supported.

This is a fork of a fork; thank you to the original authors:

• MadRabbit/express-yields
• davidbanham/express-async-errors

Usage

npm install @csnw/express-async-errors --save

Run the patch before building express().
Example:

import express from 'express';
import expressAsyncErrors from '@csnw/express-async-errors';

// Apply patch and then build express
expressAsyncErrors();
const app = express();

app.get('/delay', async (req, res) => {
  const ms = parseInt(req.query.ms);
  if (isNaN(ms)) {
    // Throw error from async request handler
    throw new Error('bad request: ms should be a number');
  }
  await new Promise((resolve) => setTimeout(resolve, ms));
  res.status(200).json({ success: true, ms });
});

app.use((err, req, res, next) => {
  // Request error handler receives the thrown error
  if (err.message?.startsWith('bad request:')) {
    res.status(400).json({ error: err.message });
    return;
  }
  res.status(500).json({ error: 'unexpected error' });
});

How Does This Work?

This is a minimalistic and unintrusive hack. Instead of patching all methods on an express Router, it wraps the Layer#handle property in one place. When a request handler is invoked, the wrapper evaluates the handler and then checks whether the returned value is a promise. If it is, a catch is appended to the promise: .catch((err) => next(err)). Your request error handler is then able to handle the error passed by next(err). If no error occurs, the catch never evaluates.

License

All code in this repository is released under the terms of the ISC license.