feat(transloco): 🎸 improve strict mode types for pipe

✅ Closes: #755

Author: Kaemmelot

libs/transloco/src/lib/tests/pipe/pipe.spec.ts

@@ -29,6 +29,21 @@ describe('TranslocoPipe', () => {
     expect(pipe.transform('title', {})).toBe('');
   });
 
+  describe('None transformable values', () => {
+    beforeEach(() => {
+      pipe = new TranslocoPipe(serviceMock, undefined, 'es', cdrMock);
+    });
+    it('should handle null', () => {
+      expect(pipe.transform(null)).toBeNull();
+    });
+    it('should handle undefined', () => {
+      expect(pipe.transform(undefined)).toBeUndefined();
+    });
+    it('should handle empty string', () => {
+      expect(pipe.transform('')).toBe('');
+    });
+  });
+
   it('should use provided language', fakeAsync(() => {
     spyOn(serviceMock, 'translate').and.callThrough();
     pipe = new TranslocoPipe(serviceMock, undefined, 'es', cdrMock);

libs/transloco/src/lib/transloco.pipe.ts

@@ -46,15 +46,36 @@ export class TranslocoPipe implements PipeTransform, OnDestroy {
     this.scopeResolver = new ScopeResolver(this.service);
   }
 
-  // null is for handling strict mode + async pipe types https://github.com/jsverse/transloco/issues/311
-  // null is for handling strict mode + optional chaining types https://github.com/jsverse/transloco/issues/488
+  // overloads for strict mode
+  transform(key: string, params?: HashMap, inlineLang?: string): string;
+  transform(
+    key: null | undefined,
+    params?: HashMap,
+    inlineLang?: string
+  ): null | undefined;
+  transform(
+    key: string | null,
+    params?: HashMap,
+    inlineLang?: string
+  ): string | null;
+  transform(
+    key: string | undefined,
+    params?: HashMap,
+    inlineLang?: string
+  ): string | undefined;
+  transform(
+    key: string | null | undefined,
+    params?: HashMap,
+    inlineLang?: string
+  ): string | null | undefined;
+
   transform(
     key?: string | null,
     params?: HashMap,
     inlineLang?: string
-  ): string {
+  ): string | null | undefined {
     if (!key) {
-      return key as any;
+      return key;
     }
 
     const keyName = params ? `${key}${JSON.stringify(params)}` : key;