Exporting webpack config as a function fails - webpack-4

Question

I'am trying to handle development and production eviroment variables in my webpack configuration (see https://webpack.js.org/guides/production/), but it fails with
WebpackOptionsValidationError: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
- configuration should be an object.
package.json
{
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1",
"build": "./node_modules/.bin/webpack",
"start": "npm run build && node server.js"
},
"devDependencies": {
//...
"webpack": "^4.20.2",
"webpack-cli": "^3.1.2",
"webpack-dev-middleware": "^3.4.0",
"webpack-hot-middleware": "^2.24.2"
}
}
webpack.config.js
const path = require('path'),
webpack = require('webpack'),
HtmlWebpackPlugin = require('html-webpack-plugin');
let config = {
entry: {
app: [
'./src/app/App.tsx', 'webpack-hot-middleware/client'
],
vendor: ['react', 'react-dom']
},
output: {
path: path.resolve(__dirname, 'dist'),
filename: 'js/[name].bundle.js'
},
// ...
}
This export is working as expected without errors or warnings
module.exports = config; // everything is fine
But this fails
module.exports = function(env, argv) { // this errors
return config;
};
There is a similiar, but unanswered question here: webpack base config as a function doesn't work
It's a very mysterious behaviour, appreciate if anyone could help!

Answer 1

Well,it is working. I didn't notice that the error takes place on a total different spot of my code.
I was doing a tutorial about HMR with webpack and express. An it's this lines of code in the express setup which causes the trouble:
server.js
const webpackConfig = require('./webpack.config');
const compiler = webpack(webpackConfig);
//...
app.use(
require('webpack-dev-middleware')(compiler, {
noInfo: true,
publicPath: webpackConfig.output.publicPath
})
);
The webpackConfig is only getting a function without being called and so it's not returning an object. So adding parenthesis is all it took to make it work.
const webpackConfig = require('./webpack.config')();
//..

Answer 2

The documentation is a bit quirky. You properly forgot the set the env variable from package.json
For instance "start": "webpack --env.prod --", in package.json will pass {prod: true} as the env variable.
Hope this helps.
More info here: https://webpack.js.org/api/cli/#environment-options