feat(toPromise): add higher-order lettable version of toPromise

src/operator/toPromise.ts

@@ -1,5 +1,5 @@
 import { Observable } from '../Observable';
-import { root } from '../util/root';
+import { toPromise as higherOrder } from '../operators/toPromise';
 
 /* tslint:disable:max-line-length */
 export function toPromise<T>(this: Observable<T>): Promise<T>;
@@ -56,20 +56,5 @@ export function toPromise<T>(this: Observable<T>, PromiseCtor: typeof Promise):
  * @owner Observable
  */
 export function toPromise<T>(this: Observable<T>, PromiseCtor?: typeof Promise): Promise<T> {
-  if (!PromiseCtor) {
-    if (root.Rx && root.Rx.config && root.Rx.config.Promise) {
-      PromiseCtor = root.Rx.config.Promise;
-    } else if (root.Promise) {
-      PromiseCtor = root.Promise;
-    }
-  }
-
-  if (!PromiseCtor) {
-    throw new Error('no Promise impl found');
-  }
-
-  return new PromiseCtor((resolve, reject) => {
-    let value: any;
-    this.subscribe((x: T) => value = x, (err: any) => reject(err), () => resolve(value));
-  });
+  return higherOrder(PromiseCtor)(this);
 }

src/operators/index.ts

@@ -72,6 +72,7 @@ export { timeout } from './timeout';
 export { timeoutWith } from './timeoutWith';
 export { timestamp } from './timestamp';
 export { toArray } from './toArray';
+export { toPromise } from './toPromise';
 export { window } from './window';
 export { windowCount } from './windowCount';
 export { windowTime } from './windowTime';

src/operators/toPromise.ts

@@ -0,0 +1,78 @@
+import { Observable } from '../Observable';
+import { root } from '../util/root';
+import { UnaryFunction } from '../interfaces';
+
+/* tslint:disable:max-line-length */
+export function toPromise<T>(): UnaryFunction<Observable<T>, Promise<T>>;
+export function toPromise<T>(PromiseCtor: typeof Promise): UnaryFunction<Observable<T>, Promise<T>>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Converts an Observable sequence to a ES2015 compliant promise.
+ *
+ * @example
+ * // Using normal ES2015
+ * let source = Rx.Observable
+ *   .of(42)
+ *   .toPromise();
+ *
+ * source.then((value) => console.log('Value: %s', value));
+ * // => Value: 42
+ *
+ * // Rejected Promise
+ * // Using normal ES2015
+ * let source = Rx.Observable
+ *   .throw(new Error('woops'))
+ *   .toPromise();
+ *
+ * source
+ *   .then((value) => console.log('Value: %s', value))
+ *   .catch((err) => console.log('Error: %s', err));
+ * // => Error: Error: woops
+ *
+ * // Setting via the config
+ * Rx.config.Promise = RSVP.Promise;
+ *
+ * let source = Rx.Observable
+ *   .of(42)
+ *   .toPromise();
+ *
+ * source.then((value) => console.log('Value: %s', value));
+ * // => Value: 42
+ *
+ * // Setting via the method
+ * let source = Rx.Observable
+ *   .of(42)
+ *   .toPromise(RSVP.Promise);
+ *
+ * source.then((value) => console.log('Value: %s', value));
+ * // => Value: 42
+ *
+ * @param {PromiseConstructor} [PromiseCtor] The constructor of the promise. If not provided,
+ * it will look for a constructor first in Rx.config.Promise then fall back to
+ * the native Promise constructor if available.
+ * @return {Promise<T>} An ES2015 compatible promise with the last value from
+ * the observable sequence.
+ * @method toPromise
+ * @owner Observable
+ */
+export function toPromise<T>(PromiseCtor?: typeof Promise): UnaryFunction<Observable<T>, Promise<T>> {
+  return (source: Observable<T>) => {
+    if (!PromiseCtor) {
+      if (root.Rx && root.Rx.config && root.Rx.config.Promise) {
+        PromiseCtor = root.Rx.config.Promise;
+      } else if (root.Promise) {
+        PromiseCtor = root.Promise;
+      }
+    }
+
+    if (!PromiseCtor) {
+      throw new Error('no Promise impl found');
+    }
+
+    return new PromiseCtor((resolve, reject) => {
+      let value: any;
+      source.subscribe((x: T) => value = x, (err: any) => reject(err), () => resolve(value));
+    });
+  };
+}