Utility function and type for compat interop API (#4673)

* compat interop util

* Create cyan-wasps-worry.md

* Update cyan-wasps-worry.md

.changeset/cyan-wasps-worry.md

@@ -0,0 +1,5 @@
+---
+"@firebase/util": patch
+---
+
+Added a utility function and type for compat interop API

packages/util/index.node.ts

@@ -37,3 +37,4 @@ export * from './src/validation';
 export * from './src/utf8';
 export * from './src/exponential_backoff';
 export * from './src/formatters';
+export * from './src/compat';

packages/util/index.ts

@@ -32,3 +32,4 @@ export * from './src/validation';
 export * from './src/utf8';
 export * from './src/exponential_backoff';
 export * from './src/formatters';
+export * from './src/compat';

packages/util/src/compat.ts

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+export interface Compat<T> {
+  _delegate: T;
+}
+
+export function getModularInstance<ExpService>(
+  service: Compat<ExpService> | ExpService
+): ExpService {
+  if (service && (service as Compat<ExpService>)._delegate) {
+    return (service as Compat<ExpService>)._delegate;
+  } else {
+    return service as ExpService;
+  }
+}

packages/util/test/compat.test.ts

@@ -0,0 +1,35 @@
+/**
+ * @license
+ * Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { expect } from 'chai';
+import { getModularInstance } from '../src/compat';
+
+describe('getModularInstance()', () => {
+  it('returns _delegate from a compat object', () => {
+    const compat = {
+      _delegate: {}
+    };
+    const modularThing = getModularInstance(compat);
+    expect(modularThing).to.eq(compat._delegate);
+  });
+
+  it('returns the object itself if it is not a compat object', () => {
+    const thing = {};
+    const modularThing = getModularInstance(thing);
+    expect(modularThing).to.eq(thing);
+  });
+});