Can't keep class names using `uglifyjs-webpack-plugin` with "mode": "production"

[lgarron]

As soon as I set module.exports.mode = "production", class names are mangled.

Some references:
https://webpack.js.org/plugins/uglifyjs-webpack-plugin/
https://github.com/webpack-contrib/uglifyjs-webpack-plugin

Absolutely nothing works to keep the class names, including this overkill config:

plugins: [
  new UglifyJSPlugin({
    sourceMap: true,
    uglifyOptions: {
      keep_classnames: true,
      "mangle": {
        keep_classnames: true
      },
      "compress": {
        keep_classnames: true
      }
    }
  })
],

Or this one:

optimization: {
  minimizer: [
    new UglifyJSPlugin({
      sourceMap: true,
      uglifyOptions: {
        keep_classnames: true,
        "mangle": {
          keep_classnames: true
        },
        "compress": {
          keep_classnames: true
        }
      }
    })
  ]
},

I know that the config is being parsed properly, because I get errors if I place keep_classnames directly in the UglifyJSPlugin config object or change keep_classnames into a typo anywhere.

[lgarron]

Okay, I think I figured it out. An output target of ES5 in tsconfig.json was probably converting classes to functions before they reached UglifyJS, at which point keep_classnames did nothing.

Changing tsconfig.json to emit ES6 seems to work, although I still need to construct the production config from scratch instead of overwriting just the UglifyJS pluging.