Dynamic imports priority fix closes #6980

packages/core/integration-tests/test/integration/dynamic-imports-high-prio/async.js

@@ -0,0 +1 @@
+export default {};

packages/core/integration-tests/test/integration/dynamic-imports-high-prio/index.js

@@ -0,0 +1 @@
+export default import('./async').then(() => document.head.children);

packages/core/integration-tests/test/javascript.js

@@ -873,6 +873,31 @@ describe('javascript', function () {
     assert.deepEqual(res, {default: 42});
   });
 
+  it('dynamic imports loaded as high-priority scripts when not all engines support esmodules natively', async function () {
+    let b = await bundle(
+      path.join(__dirname, '/integration/dynamic-imports-high-prio/index.js'),
+      {
+        defaultTargetOptions: {
+          engines: {
+            browsers: 'IE 11',
+          },
+        },
+      },
+    );
+
+    let output = await run(b);
+    let headChildren = await output.default;
+
+    assert(headChildren[1].tag === 'link');
+    assert(headChildren[1].rel === 'preload');
+    assert(headChildren[1].as === 'script');
+
+    assert(headChildren[1].href === headChildren[2].src);
+
+    assert(headChildren[2].tag === 'script');
+    assert(headChildren[2].src.match(/async\..*\.js/));
+  });
+
   it('should support bundling workers with dynamic import in both page and worker', async function () {
     let b = await bundle(
       path.join(__dirname, '/integration/worker-dynamic/index-async.js'),

packages/runtimes/js/src/helpers/browser/js-loader.js

@@ -13,6 +13,12 @@ module.exports = cacheLoader(function loadJSBundle(bundle) {
       return;
     }
 
+    var preloadLink = document.createElement('link');
+    preloadLink.href = bundle;
+    preloadLink.rel = 'preload';
+    preloadLink.as = 'script';
+    document.head.appendChild(preloadLink);
+
     var script = document.createElement('script');
     script.async = true;
     script.type = 'text/javascript';
@@ -32,6 +38,7 @@ module.exports = cacheLoader(function loadJSBundle(bundle) {
       resolve();
     };
 
+    document.getElementsByTagName('head')[0].appendChild(preloadLink);
     document.getElementsByTagName('head')[0].appendChild(script);
   });
 });