Error handler ignored when NODE_ENV=production

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 }