fromPromise

signature: fromPromise(promise: Promise, scheduler: Scheduler): Observable

Create observable from promise, emitting result.

Flattening operators can generally accept promises without wrapping!

You could also use Observable.from for the same result!

Examples

Example 1: Converting promise to observable and catching errors

( jsBin |
jsFiddle )

import { of } from 'rxjs/observable/of';
import { fromPromise } from 'rxjs/observable/fromPromise';
import { mergeMap, catchError } from 'rxjs/operators';
//example promise that will resolve or reject based on input
const myPromise = willReject => {
  return new Promise((resolve, reject) => {
    if (willReject) {
      reject('Rejected!');
    }
    resolve('Resolved!');
  });
};
//emit true, then false
const source = of(true, false);
const example = source.pipe(
  mergeMap(val =>
    fromPromise(myPromise(val)).pipe(
      //catch and gracefully handle rejections
      catchError(error => of(`Error: ${error}`))
    )
  )
);
//output: 'Error: Rejected!', 'Resolved!'
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

• fromPromise
  - Official docs
• Creation operators: from, fromArray, fromPromise
  - André Staltz
• fromPromise - Guide

Source Code:
https://github.com/ReactiveX/rxjs/blob/master/src/internal/observable/fromPromise.ts