[WIP] Build: Adopt ESM export between core.js and qunit.js

Ref qunitjs#1551

src/core/core.js

@@ -28,46 +28,55 @@ config.currentModule.suiteReport = runSuite;
 
 config._pq = new ProcessingQueue(test);
 
-const QUnit = {
+// Figure out if we're running the tests from a server or not
+const isLocal = (window && window.location && window.location.protocol === 'file:');
 
-  // Figure out if we're running the tests from a server or not
-  isLocal: (window && window.location && window.location.protocol === 'file:'),
+const begin = createRegisterCallbackFunction('begin');
+const done = createRegisterCallbackFunction('done');
+const log = createRegisterCallbackFunction('log');
+const moduleDone = createRegisterCallbackFunction('moduleDone');
+const moduleStart = createRegisterCallbackFunction('moduleStart');
+const testDone = createRegisterCallbackFunction('testDone');
+const testStart = createRegisterCallbackFunction('testStart');
 
-  // Expose the current QUnit version
-  version,
+const assert = Assert.prototype;
+
+const only = test.only;
+const skip = test.skip;
+const todo = test.todo;
+
+export {
+  hooks,
+  module,
+  start,
+  test,
+
+  only,
+  skip,
+  todo,
+
+  begin,
+  done,
+  log,
+  moduleDone,
+  moduleStart,
+  on,
+  testDone,
+  testStart,
 
   config,
-  stack,
-  urlParams,
 
+  assert,
   diff,
   dump,
   equiv,
-  reporters,
-  hooks,
   is,
-  on,
+  isLocal,
   objectType,
   onUncaughtException,
   pushFailure,
-
-  begin: createRegisterCallbackFunction('begin'),
-  done: createRegisterCallbackFunction('done'),
-  log: createRegisterCallbackFunction('log'),
-  moduleDone: createRegisterCallbackFunction('moduleDone'),
-  moduleStart: createRegisterCallbackFunction('moduleStart'),
-  testDone: createRegisterCallbackFunction('testDone'),
-  testStart: createRegisterCallbackFunction('testStart'),
-
-  assert: Assert.prototype,
-  module,
-  start,
-  test,
-
-  // alias other test flavors for easy access
-  todo: test.todo,
-  skip: test.skip,
-  only: test.only
+  reporters,
+  stack,
+  urlParams,
+  version
 };
-
-export default QUnit;

src/core/qunit.js

@@ -1,12 +1,31 @@
-import QUnit from './core.js';
+import * as QUnitCore from './core.js';
 import { initBrowser } from './browser/browser-runner.js';
 import { window, document } from './globals.js';
 import { setQUnitObject } from './start.js';
 import exportQUnit from './export.js';
 
+// The imported object from core.js, per ESM/Rollup, is frozen.
+// We support adding and replacing QUnit methods, so wrap the exported
+// object in Object.create() to create writable version. The caveat is
+// that a monkey-patched method like QUnit.test() will be reflected in
+// `import QUnit from qunit.js; QUnit.test()`
+// the change is not visible to
+// `import { test } from qunit.js`
+//
+// CJS usage is unaffected since there we assign the writeable QUnit
+// object to module.exports. The read-only QUnitCore will only
+// be exposed to ESM via `export * from './core.js';` below.
+const QUnit = Object.create(QUnitCore);
+
 setQUnitObject(QUnit);
+
 exportQUnit(QUnit);
 
 if (window && document) {
   initBrowser(QUnit, window, document);
 }
+
+// TODO: Decide how to format the CJS and ESM distribution
+// in Rollup config.
+// export * from './core.js';
+// export default QUnit;

src/core/version.js

@@ -1,2 +1,3 @@
+// Expose the current QUnit version
 // Replaced by /rollup.config.js using /build/dist-replace.js
 export default '@VERSION';