fix: provide destination property to Selector Action to infer return type of fn

Author: emyann

src/types.ts

@@ -1,4 +1,4 @@
-import { SCHEMA_OPTIONS_SYMBOL, SchemaOptions } from './morphism';
+import { SCHEMA_OPTIONS_SYMBOL, SchemaOptions } from "./morphism";
 
 /**
  * A structure-preserving object from a source data towards a target data.
@@ -35,22 +35,38 @@ export type StrictSchema<Target = any, Source = any> = {
   /** `destinationProperty` is the name of the property of the target object you want to produce */
   [destinationProperty in keyof Target]:
     | ActionString<Source>
-    | { (iteratee: Source, source: Source[], target: Target[destinationProperty]): Target[destinationProperty] }
+    | {
+        (
+          iteratee: Source,
+          source: Source[],
+          target: Target[destinationProperty]
+        ): Target[destinationProperty];
+      }
     | ActionAggregator<Source>
-    | ActionSelector<Source, Target>
+    | ActionSelector<Source, Target, destinationProperty>
     | StrictSchema<Target[destinationProperty], Source>;
 } & { [SCHEMA_OPTIONS_SYMBOL]?: SchemaOptions<Target> };
 export type Schema<Target = any, Source = any> = {
   /** `destinationProperty` is the name of the property of the target object you want to produce */
   [destinationProperty in keyof Target]?:
     | ActionString<Source>
-    | { (iteratee: Source, source: Source[], target: Target[destinationProperty]): Target[destinationProperty] }
+    | {
+        (
+          iteratee: Source,
+          source: Source[],
+          target: Target[destinationProperty]
+        ): Target[destinationProperty];
+      }
     | ActionAggregator<Source>
-    | ActionSelector<Source, Target>
+    | ActionSelector<Source, Target, destinationProperty>
     | Schema<Target[destinationProperty], Source>;
 } & { [SCHEMA_OPTIONS_SYMBOL]?: SchemaOptions<Target | any> };
 
-export type Actions<Target, Source> = ActionFunction<Target, Source> | ActionAggregator | ActionString<Target> | ActionSelector<Source>;
+export type Actions<Target, Source> =
+  | ActionFunction<Target, Source>
+  | ActionAggregator
+  | ActionString<Target>
+  | ActionSelector<Source>;
 
 /**
  * @interface ActionFunction
@@ -129,7 +145,9 @@ export type ActionString<Source> = string; // TODO: ActionString should support
  * //=> { fooAndBar: { foo: 'foo', bar: 'bar' } }
  * ```
  */
-export type ActionAggregator<T extends unknown = unknown> = T extends object ? (keyof T)[] | string[] : string[];
+export type ActionAggregator<T extends unknown = unknown> = T extends object
+  ? (keyof T)[] | string[]
+  : string[];
 /**
  * @interface ActionSelector
  * @typeparam Source Source/Input Type
@@ -157,21 +175,41 @@ export type ActionAggregator<T extends unknown = unknown> = T extends object ? (
  *```
  *
  */
-export interface ActionSelector<Source = object, R = any> {
+export interface ActionSelector<
+  Source = object,
+  Target = any,
+  TargetProperty extends keyof Target = any
+> {
   path: ActionString<Source> | ActionAggregator<Source>;
-  fn: (fieldValue: any, object: Source, items: Source, objectToCompute: R) => R;
+  fn: (
+    fieldValue: any,
+    object: Source,
+    items: Source,
+    objectToCompute: Target
+  ) => Target[TargetProperty];
 }
 
 export interface Constructable<T> {
   new (...args: any[]): T;
 }
 
-export type SourceFromSchema<T> = T extends StrictSchema<unknown, infer U> | Schema<unknown, infer U> ? U : never;
-export type DestinationFromSchema<T> = T extends StrictSchema<infer U> | Schema<infer U> ? U : never;
+export type SourceFromSchema<T> = T extends
+  | StrictSchema<unknown, infer U>
+  | Schema<unknown, infer U>
+  ? U
+  : never;
+export type DestinationFromSchema<T> = T extends
+  | StrictSchema<infer U>
+  | Schema<infer U>
+  ? U
+  : never;
 
 export type ResultItem<TSchema extends Schema> = DestinationFromSchema<TSchema>;
 
-export interface Mapper<TSchema extends Schema | StrictSchema, TResult = ResultItem<TSchema>> {
+export interface Mapper<
+  TSchema extends Schema | StrictSchema,
+  TResult = ResultItem<TSchema>
+> {
   (data?: SourceFromSchema<TSchema>[] | null): TResult[];
   (data?: SourceFromSchema<TSchema> | null): TResult;
 }