test: add attempt to test the new feature added

Since there's no other way to check TypeScript types within this project
without addind some 3rd-party lib, I've made this approach: let TSC
complains about typings while running the npm-script `test:integration`.

tests/jest-e2e.json

@@ -1,4 +1,9 @@
 {
+  "globals": {
+    "ts-jest": {
+      "tsconfig": "./tests/tsconfig.json"
+    }
+  },
   "moduleFileExtensions": ["js", "json", "ts"],
   "rootDir": ".",
   "testEnvironment": "node",

tests/src/app.module.ts

@@ -7,15 +7,44 @@ import { ConfigService } from '../../lib/config.service';
 import databaseConfig from './database.config';
 import nestedDatabaseConfig from './nested-database.config';
 
+type Config = {
+  database: ConfigType<typeof databaseConfig> & {
+    driver: ConfigType<typeof nestedDatabaseConfig>
+  };
+};
+
 @Module({})
 export class AppModule {
   constructor(
     private readonly configService: ConfigService,
+    // The following is the same object as above but narrowing its types
+    private readonly configServiceNarrowed: ConfigService<Config, true>,
     @Optional()
     @Inject(databaseConfig.KEY)
     private readonly dbConfig: ConfigType<typeof databaseConfig>,
   ) {}
 
+  /**
+   * This method is not meant to be used anywhere! It just here for testing
+   * types defintions while runnig test suites (in some sort).
+   * If some typings doesn't follows the requirements, Jest will fail due to
+   * TypeScript errors.
+   */
+  private noop(): void {
+    // Arrange
+    const identityString = (v: string) => v;
+    const identityNumber = (v: number) => v;
+    // Act
+    const knowConfig = this.configServiceNarrowed.get<Config['database']>('database');
+    // Assert
+    // We don't need type assertions bellow anymore since `knowConfig` is not
+    // expected to be `undefined` beforehand.
+    identityString(knowConfig.host);
+    identityNumber(knowConfig.port);
+    identityString(knowConfig.driver.host);
+    identityNumber(knowConfig.driver.port);
+  }
+
   static withCache(): DynamicModule {
     return {
       module: AppModule,

tests/tsconfig.json

@@ -0,0 +1,14 @@
+{
+  "extends": "../tsconfig.json",
+  "compilerOptions": {
+    "rootDir": "../",
+    "strictNullChecks": true
+  },
+  "include": [
+    "**/*.spec.ts",
+  ],
+  "exclude": [
+    "node_modules",
+    "dist"
+  ]
+}