[monorepo] prepare per-project queues (#35386)

GitOrigin-RevId: ab020603ec04c705d08be466563adac0ce30bc04

services/document-updater/app/js/UpdateKeys.js

@@ -1,10 +1,28 @@
 // TODO: This file was created by bulk-decaffeinate.
 // Fix any style issues and re-enable lint.
 module.exports = {
+  /**
+   * Build a legacy `project_id:doc_id` dispatch marker.
+   *
+   * @param {string} projectId
+   * @param {string} docId
+   * @return {string}
+   */
   combineProjectIdAndDocId(projectId, docId) {
     return `${projectId}:${docId}`
   },
+
+  /**
+   * Split a dispatch marker into project id and doc id. A marker is either
+   * the legacy `project_id:doc_id` or, after the queue migration, a bare
+   * `project_id`. Project and doc ids never contain a colon, so splitting is
+   * unambiguous; for the bare form docId is undefined.
+   *
+   * @param {string} projectAndDocId
+   * @return {[string, string | undefined]}
+   */
   splitProjectIdAndDocId(projectAndDocId) {
-    return projectAndDocId.split(':')
+    const [projectId, docId] = projectAndDocId.split(':')
+    return [projectId, docId]
   },
 }