feat(): let ConfigService type know about schema validation

Closes #668

lib/config.service.ts

@@ -7,7 +7,7 @@ import {
   CONFIGURATION_TOKEN,
   VALIDATED_ENV_PROPNAME,
 } from './config.constants';
-import { NoInferType, Path, PathValue } from './types';
+import { NoInferType, Path, PathValue, ExcludeUndefinedIf } from './types';
 
 export interface ConfigGetOptions {
   /**
@@ -19,7 +19,10 @@ 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 +45,7 @@ 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 +55,7 @@ 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").

lib/types/exclude-undefined-if.type.ts

@@ -0,0 +1,10 @@
+/**
+ * `ExcludeUndefinedIf<ExcludeUndefined, T>
+ *
+ * If `ExcludeUndefined` is `true`, remove `undefined` from `T`.
+ * Otherwise, constructs the type `T` with `undefined`.
+ */
+export type ExcludeUndefinedIf<
+  ExcludeUndefined extends boolean,
+  T,
+> = ExcludeUndefined extends true ? Exclude<T, undefined> : T | undefined;

lib/types/index.ts

@@ -2,3 +2,4 @@ export * from './config-object.type';
 export * from './config.type';
 export * from './no-infer.type';
 export * from './path-value.type';
+export * from './exclude-undefined-if.type';