added config to only support browsers supporting esmodules

Author: bennobuilder

packages/babel.config.js

@@ -2,8 +2,24 @@
 export default function getBabelConfig() {
   return {
     ignore: ['./node_modules'],
-    presets: ['@babel/preset-env'],
-    // https://stackoverflow.com/questions/33527653/babel-6-regeneratorruntime-is-not-defined
-    plugins: ['@babel/transform-runtime'],
+    presets: [
+      [
+        '@babel/preset-env',
+        {
+          targets: {
+            // Only targeting browsers supporting ES Modules (https://babeljs.io/docs/en/babel-preset-env)
+            // Why?: https://github.com/babel/babel/issues/9849#issuecomment-592668815
+            esmodules: true,
+          },
+        },
+      ],
+    ],
+    // https://stackoverflow.com/questions/53558916/babel-7-referenceerror-regeneratorruntime-is-not-defined/61517521#61517521
+    // Unfortunately this plugin requires an external (prod) dependency '@babel/runtime'.
+    // However the required dependency is also modular and shrinks the bundle size in general due to function reuses.
+    // (See: https://babeljs.io/docs/en/babel-runtime)
+    // Note: Required when using "babelHelpers: 'runtime'" in the 'rollup.config.default.js'
+    //       and to support browsers that do not support ES Modules like 'IE11'
+    // plugins: ['@babel/transform-runtime'],
   };
 }

packages/rollup.config.default.js

@@ -14,7 +14,11 @@ export const fileExtensions = ['.ts'];
 function createBabelConfig() {
   // https://github.com/rollup/plugins/tree/master/packages/babel#running-babel-on-the-generated-code
   return babel({
-    babelHelpers: 'runtime',
+    // 'babelHelpers' option 'runtime' is recommended for libraries.
+    // However using the 'runtime' options requires an external (prod) dependency ('@babel/runtime') which we want to avoid at this point in time.
+    // (See: https://github.com/rollup/plugins/tree/master/packages/babel#babelhelpers)
+    // Therefore we decided against it and only support browsers that support ES modules.
+    babelHelpers: 'bundled',
     comments: false,
     extensions: fileExtensions, // https://github.com/rollup/rollup-plugin-babel/issues/255
     ...getBabelConfig(),
@@ -109,6 +113,7 @@ export function createCommonJSConfig(config) {
     output: {
       file: config.output,
       format: 'cjs',
+      sourcemap: true,
     },
     external: config.external,
     plugins: [