Webpack: Unknown argument: mode / configuration has an unknown property 'mode'

For users having to support legacy software (hi Debian folks) here is the patch from v4 to v3 support.

[...]
+const webpack = require('webpack');
[...]
     devtool: 'source-map',
-    mode: 'production',
+    // mode: 'production', // Webpack >= 4
[...]
+    plugins: [
+        new webpack.DefinePlugin({
+            'process.env.NODE_ENV': JSON.stringify('production')
+        })
+    ],
[...]

Ok, the problem was that I had an older webpack installed globally, I think...

I had some old version running globally on my system. Once I removed them things started working for me. I ran: which webpack to see where they were coming from. I had npm and ruby gems both installed so I removed them both.

Uninstall with NPM

npm un -g webpack

Uninstall Ruby Gems

It may prompt you which version to uninstall. I choose all of them.

gem uninstall webpacker

I had some installed with ruby gems so I had to run gem uninstall webpacker I ended up removing all of them. Then I had to run bundle install again. It can be useful to run which webpack to determine where it's globally installed. I would recommend globally uninstalling it for ruby gems and npm. To uninstall with npm: npm un -g webpack