Fix event tests. Add developer notes.

Author: kinyoklion

packages/shared/sdk-client/__tests__/LDClientImpl.test.ts

@@ -268,7 +268,9 @@ describe('sdk-client object', () => {
     const carContext2: LDContext = { kind: 'car', key: 'test-car-2' };
     await ldc.identify(carContext2);
 
-    expect(emitter.listenerCount('change')).toEqual(1);
+    // No default listeners. This is important for clients to be able to determine if there are
+    // any listeners and act on that information.
+    expect(emitter.listenerCount('change')).toEqual(0);
     expect(emitter.listenerCount('error')).toEqual(1);
   });
 

packages/shared/sdk-client/src/LDEmitter.ts

@@ -2,6 +2,12 @@ import { LDLogger } from '@launchdarkly/js-sdk-common';
 
 export type EventName = 'error' | 'change';
 
+/**
+ * Implementation Note: There should not be any default listeners for change events in a client
+ * implementation. Default listeners mean a client cannot determine when there are actual
+ * application developer provided listeners. If we require default listeners, then we should add
+ * a system to allow listeners which have counts independent of the primary listener counts.
+ */
 export default class LDEmitter {
   private listeners: Map<EventName, Function[]> = new Map();