refactor: Return ReactSDKClient interface from createInstance instead of implementation class (#148)

### Summary
Return type of createInstance was OptimizelyReactSDKClient which is the implementation class. Changed it to the ReactSDKClient interface instead.

### Test Plan

- Manually tested thoroughly.
- All existing unit tests pass.

### Issues
#49

Author: shaharyar123

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
+### Enhancements
+- fixed issue [#49](https://github.com/optimizely/react-sdk/issues/49): Return type of `createInstance` was `OptimizelyReactSDKClient` which is the implementation class. Changed it to the `ReactSDKClient` interface instead ([#148](https://github.com/optimizely/react-sdk/pull/148)).
+
 ## [2.8.0] - January 26, 2022
 
 ### New Features

src/client.spec.ts

@@ -18,6 +18,10 @@ jest.mock('@optimizely/optimizely-sdk');
 import * as optimizely from '@optimizely/optimizely-sdk';
 
 import { createInstance, OnReadyResult, ReactSDKClient } from './client';
+interface MockedReactSDKClient extends ReactSDKClient {
+  client: optimizely.Client;
+  initialConfig: optimizely.Config;
+}
 
 describe('ReactSDKClient', () => {
   const config: optimizely.Config = {
@@ -79,7 +83,7 @@ describe('ReactSDKClient', () => {
 
   it('provides the initial config object via the initialConfig property', () => {
     const instance = createInstance(config);
-    expect(instance.initialConfig).toEqual(config);
+    expect((instance as MockedReactSDKClient).initialConfig).toEqual(config);
   });
 
   it('provides a default user object', () => {
@@ -94,7 +98,7 @@ describe('ReactSDKClient', () => {
     const instance = createInstance(config);
     expect(createInstanceSpy).toBeCalledTimes(1);
     expect(createInstanceSpy.mock.results[0].type).toBe('return');
-    expect(createInstanceSpy.mock.results[0].value).toBe(instance.client);
+    expect(createInstanceSpy.mock.results[0].value).toBe((instance as MockedReactSDKClient).client);
   });
 
   it('adds react-sdk clientEngine and clientVersion to the config, and passed the config to createInstance', () => {
@@ -109,7 +113,7 @@ describe('ReactSDKClient', () => {
 
   it('provides access to the underlying client notificationCenter', () => {
     const instance = createInstance(config);
-    expect(instance.notificationCenter).toBe(instance.client.notificationCenter);
+    expect(instance.notificationCenter).toBe((instance as MockedReactSDKClient).client.notificationCenter);
   });
 
   describe('onReady', () => {

src/client.ts

@@ -168,6 +168,8 @@ export interface ReactSDKClient extends Omit<optimizely.Client, 'createUserConte
   removeAllForcedDecisions(): boolean;
 
   removeForcedDecision(decisionContext: optimizely.OptimizelyDecisionContext): boolean;
+
+  getForcedDecision(decisionContext: optimizely.OptimizelyDecisionContext): optimizely.OptimizelyForcedDecision | null;
 }
 
 export const DEFAULT_ON_READY_TIMEOUT = 5000;
@@ -921,6 +923,6 @@ class OptimizelyReactSDKClient implements ReactSDKClient {
   }
 }
 
-export function createInstance(config: optimizely.Config): OptimizelyReactSDKClient {
+export function createInstance(config: optimizely.Config): ReactSDKClient {
   return new OptimizelyReactSDKClient(config);
 }