fix(reduce): type definitions overloads for TypeScript are now in pro…

…per order (#2523)

* Change order of reduce overload type definitions

Returning type T takes precedence over returning type T[].

* test(reduce): add type tests for arrays

Add tests for reducing T[] to T[] (e.g. concatenating arrays into one array) and for reducing T to T[] (e.g. pushing items into an array).

spec/operators/reduce-spec.ts

@@ -316,6 +316,33 @@ describe('Observable.prototype.reduce', () => {
     });
   });
 
+  it('should accept T typed reducers when T is an array', () => {
+    type(() => {
+      let a: Rx.Observable<number[]>;
+      const reduced = a.reduce((acc, value) => {
+        return acc.concat(value);
+      }, []);
+
+      reduced.subscribe(rs => {
+        rs[0].toExponential();
+      });
+    });
+  });
+
+  it('should accept R typed reduces when R is an array of T', () => {
+    type(() => {
+      let a: Rx.Observable<number>;
+      const reduced = a.reduce((acc, value) => {
+        acc.push(value);
+        return acc;
+      }, []);
+
+      reduced.subscribe(rs => {
+        rs[0].toExponential();
+      });
+    });
+  });
+
   it('should accept R typed reducers when R is assignable to T', () => {
     type(() => {
       let a: Rx.Observable<{ a?: number; b?: string }>;

src/operator/reduce.ts

@@ -3,8 +3,8 @@ import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
 
 /* tslint:disable:max-line-length */
-export function reduce<T>(this: Observable<T>, accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): Observable<T[]>;
 export function reduce<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
+export function reduce<T>(this: Observable<T>, accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): Observable<T[]>;
 export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed: R): Observable<R>;
 /* tslint:enable:max-line-length */