feat: let ConfigService type know about schema validation

lib/config.service.ts

@@ -9,6 +9,17 @@ import {
 } from './config.constants';
 import { NoInferType, Path, PathValue } from './types';
 
+/**
+ * `ExcludeUndefinedIf<ExcludeUndefined, T>
+ *
+ * If `ExcludeUndefined` is `true`, remove `undefined` from `T`.
+ * Otherwise, constructs the type `T` with `undefined`.
+ */
+type ExcludeUndefinedIf<
+  ExcludeUndefined extends boolean,
+  T,
+> = ExcludeUndefined extends true ? Exclude<T, undefined> : T | undefined;
+
 export interface ConfigGetOptions {
   /**
    * If present, "get" method will try to automatically
@@ -19,7 +30,10 @@ export interface ConfigGetOptions {
 }
 
 @Injectable()
-export class ConfigService<K = Record<string, unknown>> {
+export class ConfigService<
+  K = Record<string, unknown>,
+  WasValidated extends boolean = false,
+> {
   private set isCacheEnabled(value: boolean) {
     this._isCacheEnabled = value;
   }
@@ -42,7 +56,7 @@ export class ConfigService<K = Record<string, unknown>> {
    * based on property path (you can use dot notation to traverse nested object, e.g. "database.host").
    * @param propertyPath
    */
-  get<T = any>(propertyPath: keyof K): T | undefined;
+  get<T = any>(propertyPath: keyof K): ExcludeUndefinedIf<WasValidated, T>;
   /**
    * Get a configuration value (either custom configuration or process environment variable)
    * based on property path (you can use dot notation to traverse nested object, e.g. "database.host").
@@ -52,7 +66,7 @@ export class ConfigService<K = Record<string, unknown>> {
   get<T = K, P extends Path<T> = any, R = PathValue<T, P>>(
     propertyPath: P,
     options: ConfigGetOptions,
-  ): R | undefined;
+  ): ExcludeUndefinedIf<WasValidated, R>;
   /**
    * Get a configuration value (either custom configuration or process environment variable)
    * based on property path (you can use dot notation to traverse nested object, e.g. "database.host").

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"
+  ]
+}