misc: make copyUntypedFiles work for watch mode (#7445)

admin/scripts/copyUntypedFiles.mjs

@@ -0,0 +1,36 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import fs from 'fs-extra';
+import path from 'path';
+import chokidar from 'chokidar';
+
+const srcDir = path.join(process.cwd(), 'src');
+const libDir = path.join(process.cwd(), 'lib');
+
+const ignoredPattern = /(?:__tests__|\.tsx?$)/;
+
+async function copy() {
+  await fs.copy(srcDir, libDir, {
+    filter(testedPath) {
+      return !ignoredPattern.test(testedPath);
+    },
+  });
+}
+
+if (process.argv.includes('--watch')) {
+  const watcher = chokidar.watch(srcDir, {
+    ignored: ignoredPattern,
+    ignoreInitial: true,
+    persistent: true,
+  });
+  ['add', 'change', 'unlink', 'addDir', 'unlinkDir'].forEach((event) =>
+    watcher.on(event, copy),
+  );
+} else {
+  await copy();
+}