docs: update config.md

docs/config.md

@@ -8,6 +8,8 @@ Nestjs provides excellent configuration support, thus your application can read
 
 In this post, we will move our hard-coded configuration we've used in the previous posts to a central place and organize them with the NestJS configuration facilities.
 
+## Introduce to ConfigModule
+
 First of all, install `@nestjs/config` package.
 
 ```bash
@@ -51,6 +53,8 @@ ConfigModule.forRoot({
 
 For the container deployment or cloud deployment, setup configuration in a config server or as environment variables or  K8s *ConfigMap* is more popular. 
 
+## Externalizing application configurations
+
 Personally, I prefer use a default hard-coded configuration in the development phase, and use environmental variables to override it in the production.
 
 Nestjs also support load custom configuration where it can read configurations from the environment variables freely.
@@ -201,3 +205,39 @@ services:
 
 We will start a new topic of deployment in the further posts.
 
+
+
+## Testing configurations
+
+An example of `jwt-config.spec.ts`.
+
+```typescript
+import { ConfigModule, ConfigType } from '@nestjs/config';
+import { TestingModule, Test } from '@nestjs/testing';
+import jwtConfig from './jwt.config';
+
+describe('jwtConfig', () => {
+  let config: ConfigType<typeof jwtConfig>;
+  beforeEach(async () => {
+    const module: TestingModule = await Test.createTestingModule({
+      imports: [ConfigModule.forFeature(jwtConfig)],
+    }).compile();
+
+    config = module.get<ConfigType<typeof jwtConfig>>(jwtConfig.KEY);
+  });
+
+  it('should be defined', () => {
+    expect(jwtConfig).toBeDefined();
+  });
+
+  it('should contains expiresIn and secret key', async () => {
+    expect(config.expiresIn).toBe('3600s');
+    expect(config.secretKey).toBe('rzxlszyykpbgqcflzxsqcysyhljt');
+  });
+});
+```
+
+Grab [the source codes from my github](https://github.com/hantsy/nestjs-sample), switch to branch [feat/config](https://github.com/hantsy/nestjs-sample/blob/feat/config).
+
+
+