Webpack additional compilation pass

my-webpack-plugin.js

const PLUGIN_NAME = 'MY_WEBPACK_PLUGIN';

class MyWebpackPlugin {
  constructor() {
    this.cssReady = false;
    this.cssFiles = [];
  }
  apply(compiler) {
    compiler.hooks.watchRun.tap(PLUGIN_NAME, () => {
      this.cssReady = false;
    });

    compiler.hooks.compilation.tap(
      PLUGIN_NAME,
      (compilation, { normalModuleFactory }) => {
        // copied from DefinePlugin
        compilation.dependencyFactories.set(ConstDependency, new NullFactory());
        compilation.dependencyTemplates.set(
          ConstDependency,
          new ConstDependency.Template()
        );

        const handler = parser => {
          parser.hooks.expression
            .for('CSS_FILES')
            .tap(PLUGIN_NAME, expr => {
              return ParserHelpers.toConstantDependency(
                parser,
                JSON.stringify(this.cssFiles)
              )(expr);
            });
        };

        normalModuleFactory.hooks.parser
          .for('javascript/auto')
          .tap(PLUGIN_NAME, handler);
        normalModuleFactory.hooks.parser
          .for('javascript/dynamic')
          .tap(PLUGIN_NAME, handler);
        normalModuleFactory.hooks.parser
          .for('javascript/esm')
          .tap(PLUGIN_NAME, handler);
        // END copied from DefinePlugin

        compilation.hooks.needAdditionalPass.tap(PLUGIN_NAME, () => {
          if (!this.cssReady) {
            this.cssReady = true;
            return true;
          }
        });
      }
    );

    compiler.hooks.emit.tap(PLUGIN_NAME, compilation => {
      if (!this.cssReady) {
        this.cssFiles = Object.keys(compilation.assets)
          .filter(file => file.endsWith('.css'))
          .map(file => compilation.options.output.publicPath + file);
      }
    });
  }
}