Error handler ignored when NODE_ENV=production - json

Question

I am building a simple REST API with Node/Express, and I'm having a hard time when I deploy it to production. When NODE_ENV=development, everything works as expected. I get back the JSON error and the correct status code. When NODE_ENV=production, I only get back an HTML page with the default error message and nothing else. I can read the status code, but I need to have access to the full JSON payload to identify the errors better. This is my code:
import Promise from 'bluebird'; // eslint-disable-line no-unused-vars
import express from 'express';
import config from './config';
import routes from './routes';
import { errorMiddleware, notFoundMiddleware } from './middlewares/error.middleware';
import mongoose from './config/mongoose.config';
// create app
const app = express();
(async () => {
// connect to mongoose
await mongoose.connect();
// pretty print on dev
if (process.env.NODE_ENV !== 'production') {
app.set('json spaces', 2);
}
// apply express middlewares
app.use(express.json());
// register v1 routes
app.use('/v1', routes);
// catch errors
app.use(notFoundMiddleware);
app.use(errorMiddleware);
// start server
app.listen(config.port, () => console.info(`server started on port ${config.port}`));
})();
export default app;
This is the notFoundMiddleware:
export default (req, res, next) => next(new Error('Not Found'));
This is the errorMiddleware:
const errorMiddleware = (err, req, res, next) => {
console.log('test'); // this works in development, but not in production
const error = {
status: err.status,
message: err.message
};
if (err.errors) {
error.errors = err.errors;
}
if (process.env.NODE_ENV !== 'production' && err.stack) {
error.stack = err.stack;
}
return res.status(error.status || 500).send({ error });
};

Answer 1

If you are runing on production server, try to use some logging provider like "papertrailapp" to see the error occurs in your app.

Answer 2

I've just stumbled upon the same problem. It turned out it's caused by a transpiler optimization applied when building production bundle - this one: https://babeljs.io/docs/en/babel-plugin-minify-dead-code-elimination
Express' error handlers should have the signature (err, req, res, next) => { ... } (be of arity 4). In your example next is not used anywhere in errorMiddleware function body and thus it gets eliminated (optimized-out) from function signature in production code.
Solution:
use keepFnArgs: true plugin option - possibly through https://webpack.js.org/plugins/babel-minify-webpack-plugin/ webpack configuration:
var MinifyPlugin = require("babel-minify-webpack-plugin")
module.exports = {
// ...
optimization: {
minimizer: [
new MinifyPlugin({
deadcode: {
keepFnArgs: true,
},
}, {}),
],
}
// ...
}
or alternatively pretend in your code that this argument is used:
const errMiddleware = (err, req, res, _next) => {
// ... your code ...
// ...
// cheat here:
_next
}