rxjs/src/operators/toPromise.ts

Reactive Extensions for modern JavaScript

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));
    });
  };
}