Hack: leave .mjs as esm

build-system/common/esbuild-babel.js

@@ -83,17 +83,53 @@ function getEsbuildBabelPlugin(
  build.onLoad({filter: /\.[cm]?js$/, namespace: ''}, async (file) => {
  const filename = file.path;
  const {contents, hash} = await batchedRead(filename, optionsHash);
- const transformed = await transformContents(filename, contents, hash, {
-  ...babelOptions,
+
+ const transformed = await transformContents(
  filename,
- filenameRelative: path.basename(filename),
- });
+ contents,
+ hash,
+ getFileBabelOptions(babelOptions, filename)
+ );
  return {contents: transformed};
  });
  },
  };
 }
 
+/**
+ * @param {!Object} babelOptions
+ * @param {string} filename
+ * @return {!Object}
+ */
+function getFileBabelOptions(babelOptions, filename) {
+ // Patch for leaving .mjs files as esm, since esbuild will break when trying
+ // to process a .mjs file that contains CJS exports. This function is called after
+ // babel.loadOptions, therefore all of the plugins from preset-env have already been applied.
+ // and must be disabled individually.
+ if (filename.endsWith('.mjs')) {
+ const plugins = [...babelOptions.plugins];
+ const toRemove = [
+ 'transform-modules-commonjs',
+ 'proposal-dynamic-import',
+ 'syntax-dynamic-import',
+ 'proposal-export-namespace-from',
+ 'syntax-export-namespace-from',
+ ];
+ for (const plugin of toRemove) {
+ const pluginIndex = plugins.findIndex(({key}) => key === plugin);
+ plugins.splice(pluginIndex, 1);
+ }
+
+ babelOptions = {...babelOptions, plugins};
+ }
+
+ return {
+ ...babelOptions,
+ filename,
+ filenameRelative: path.basename(filename),
+ };
+}
+
 module.exports = {
  getEsbuildBabelPlugin,
 };