Allow a string argument when constructing an App

CHANGELOG.md

@@ -5,7 +5,7 @@ NOTE: Support for syncing with realm.cloud.io and/or Realm Object Server has bee
 NOTE: This version bumps the Realm file format to version 11. It is not possible to downgrade to earlier versions. Older files will automatically be upgraded to the new file format. Files created by Realm JavaScript prior to v1.0.0, might not be upgradeable. Only [Realm Studio 10.0.0](https://github.com/realm/realm-studio/releases/tag/v10.0.0-beta.1) or later will be able to open the new file format.
 
 ### Enhancements
-* None.
+* Adding the possibility of passing a string argument when constructing an App. ([#3082](https://github.com/realm/realm-js/pull/3082))
 
 ### Fixed
 * A crash when calling `user.apiKeys.fetchAll()`

docs/sync.js

@@ -89,10 +89,10 @@
     /**
      * Creates a new app and connects to a MongoDB Realm instance.
      *
-     * @param {Realm.App~AppConfiguration} config - The configuration of the app.
+     * @param {(Realm.App~AppConfiguration|string)} configOrId - The configuration of the app or a string app id.
      * @throws If no app id is provided.
      */
-    constructor(config) { }
+    constructor(configOrId) { }
 
     /**
      * Logs in a user.

src/js_app.hpp

@@ -139,6 +139,10 @@ void AppClass<T>::constructor(ContextType ctx, ObjectType this_object, Arguments
                 config.local_app_version = util::Optional<std::string>(Value::validated_to_string(ctx, config_app_version_value, "version"));
             }
         }
+    } else if (Value::is_string(ctx, args[0])) {
+        config.app_id = Value::validated_to_string(ctx, args[0]);
+    } else {
+        throw std::runtime_error("Expected either a configuration object or an app id string.");
     }
 
     Protected<typename T::GlobalContext> protected_ctx(Context<T>::get_global_context(ctx));

tests/js/app-tests.js

@@ -53,6 +53,28 @@ module.exports = {
         TestCase.assertInstanceOf(app, Realm.App);
     },
 
+    testNewAppFromString() {
+        let app = new Realm.App(config.id);
+        TestCase.assertInstanceOf(app, Realm.App);
+        TestCase.assertEqual(app.id, config.id);
+    },
+
+    testNewAppFromUndefined() {
+        const error = TestCase.assertThrows(() =>  new Realm.App());
+        TestCase.assertEqual(
+            error.message,
+            'Invalid arguments: 1 expected, but 0 supplied.',
+        );
+    },
+
+    testNewAppFromOther() {
+        const error = TestCase.assertThrows(() => new Realm.App(1234));
+        TestCase.assertEqual(
+            error.message,
+            'Expected either a configuration object or an app id string.',
+        );
+    },
+
     async testInvalidServer() {
         const conf = {
             id: 'smurf',