fix: Destination type is properly inferred from source and provided class

Author: emyann

src/morphism.ts

@@ -148,7 +148,17 @@ function morphism<TSchema extends Schema, TDestination>(
   type: Constructable<TDestination>
 ): Mapper<TSchema, TDestination>; // morphism({}, null, T) => mapper(S) => T
 
-function morphism<TSchema extends Schema, Target>(schema: TSchema, items: SourceFromSchema<TSchema>, type: Constructable<Target>): Target; // morphism({}, {}, T) => T
+function morphism<
+  TSchema = Schema<DestinationFromSchema<Schema>, SourceFromSchema<Schema>>,
+  Target = never,
+  Source extends SourceFromSchema<TSchema> = SourceFromSchema<TSchema>
+>(schema: TSchema, items: Source, type: Constructable<Target>): Target; // morphism({}, {}, T) => T
+
+function morphism<
+  TSchema = Schema<DestinationFromSchema<Schema>, SourceFromSchema<Schema>>,
+  Target = never,
+  Source extends SourceFromSchema<TSchema> = SourceFromSchema<TSchema>
+>(schema: TSchema, items: Source[], type: Constructable<Target>): Target[]; // morphism({}, [], T) => T[]
 
 function morphism<Target, Source, TSchema extends Schema<Target, Source>>(
   schema: TSchema,

src/typescript.spec.ts

@@ -161,6 +161,29 @@ describe('Typescript', () => {
       morphism<StrictSchema<D1, S1>>({ a: ({ _a }) => _a.toString() });
       morphism<StrictSchema<D1, S1>>({ a: ({ _a }) => _a.toString() });
     });
+
+    it('shoud infer result type from source when a class is provided', () => {
+      class Source {
+        constructor(public id: number, public ugly_field: string) {}
+      }
+
+      class Destination {
+        constructor(public id: number, public field: string) {}
+      }
+
+      const source = [new Source(1, 'abc'), new Source(1, 'def')];
+
+      const schema: StrictSchema<Destination, Source> = {
+        id: 'id',
+        field: 'ugly_field'
+      };
+      const expected = [new Destination(1, 'abc'), new Destination(1, 'def')];
+
+      const result = morphism(schema, source, Destination);
+      result.forEach((item, idx) => {
+        expect(item).toEqual(expected[idx]);
+      });
+    });
   });
 
   describe('Morphism Function Type Checking', () => {