javascript - Core-JS 3 Babel results in mini-css-extract-plugin error

Question

Following on from this question I have set up my Webpack 4 config to handle babel-loader like this

{
  module : {
    rules : [{
      test : /.js$/,
      // Some module should not be transpiled by Babel
      // See https://github.com/zloirock/core-js/issues/743#issuecomment-572074215
      exclude: ['/node_modules/', /core-js/, /webpack/buildin/, /@babel/runtime-corejs3/],
      loader : "babel-loader",
      options : {
        babelrc : false,
        // Fixes "TypeError: __webpack_require__(...) is not a function"
        // https://github.com/webpack/webpack/issues/9379#issuecomment-509628205
        // https://babeljs.io/docs/en/options#sourcetype
        sourceType : "unambiguous",
        presets : [
          ["@babel/preset-env", {
            // Webpack supports ES Modules out of the box and therefore doesn’t require
            // import/export to be transpiled resulting in smaller builds, and better tree
            // shaking. See https://webpack.js.org/guides/tree-shaking/#conclusion
            modules : false,
            // Adds specific imports for polyfills when they are used in each file.
            // Take advantage of the fact that a bundler will load the polyfill only once.
            useBuiltIns : "usage",
            corejs : {
              version : "3",
              proposals : true
            }
          }]
        ]
      }
    }
  }
}

When I run this in the browser I get this error which I do not understand:

Module build failed (from ./node_modules/mini-css-extract-plugin/dist/loader.js):
TypeError: Cannot convert undefined or null to object

What is the fix for this? Following this question I have searched everywhere for code starting import 'core-js but cannot see it anywhere so have reached a dead end.

Answer 1

I had a similar problem and the reason was that Babel was transpiling my css-loader webpack plugin. Look here: MiniCssExtractPlugin error on entry point build