Use ES modules and dynamic import for Webpack latest builds (#16620)

.browserslistrc

@@ -10,6 +10,12 @@ supports es6-module-dynamic-import
 not Safari < 13
 not iOS < 13
 
+# Exclude KaiOS, QQ, and UC browsers due to lack of sufficient feature support data
+# Babel ignores these automatically, but we need here for Webpack to output ESM with dynamic imports
+not KaiOS > 0
+not QQAndroid > 0
+not UCAndroid > 0
+
 # Exclude unsupported browsers
 not dead
 

build-scripts/bundle.cjs

@@ -77,6 +77,7 @@ module.exports.htmlMinifierOptions = {
 module.exports.terserOptions = ({ latestBuild, isTestBuild }) => ({
   safari10: !latestBuild,
   ecma: latestBuild ? 2015 : 5,
+  module: latestBuild,
   format: { comments: false },
   sourceMap: !isTestBuild,
 });

build-scripts/webpack.cjs

@@ -41,7 +41,7 @@ const createWebpackConfig = ({
   return {
     name,
     mode: isProdBuild ? "production" : "development",
-    target: ["web", latestBuild ? "es2017" : "es5"],
+    target: `browserslist:${latestBuild ? "modern" : "legacy"}`,
     // For tests/CI, source maps are skipped to gain build speed
     // For production, generate source maps for accurate stack traces without source code
     // For development, generate "cheap" versions that can map to original line numbers
@@ -163,6 +163,7 @@ const createWebpackConfig = ({
       },
     },
     output: {
+      module: latestBuild,
       filename: ({ chunk }) =>
         !isProdBuild || isStatsBuild || dontHash.has(chunk.name)
           ? "[name].js"
@@ -196,7 +197,7 @@ const createWebpackConfig = ({
           : undefined,
     },
     experiments: {
-      topLevelAwait: true,
+      outputModule: true,
     },
   };
 };

test/webpack.config.js

@@ -1,8 +1,13 @@
 import webpack from "../build-scripts/webpack.cjs";
 
-export default webpack.createAppConfig({
+const config = webpack.createAppConfig({
   isProdBuild: false,
   latestBuild: true,
   isStatsBuild: false,
   isTestBuild: true,
 });
+
+// instant-mocha forces a CJS library, so cannot output ESM
+config.output.module = false;
+
+export default config;