CODEWITHSUNDEEP
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Corey Ogburn
Corey Ogburn

Reputation: 24717

Promise.all behavior with RxJS Observables?

In Angular 1.x I would sometimes need to make multiple http requests and do something with all the responses. I would throw all the promises in an array and call Promise.all(promises).then(function (results) {...}).

Angular 2 best practices seem to point towards the use of RxJS's Observable as a replacement to promises in http requests. If I have two or more different Observables created from http requests, is there an equivalent to Promise.all()?

Upvotes: 122

Views: 94298

Answers (4)

user3743222
user3743222

Reputation: 18665

The more straightforward alternative for emulating Promise.all is to use the forkJoin operator (it starts all observables in parallel and join their last elements):

A bit out of scope, but in case it helps, on the subject of chaining promises, you can use a simple flatMap : Cf. RxJS Promise Composition (passing data)

Upvotes: 99

arcseldon
arcseldon

Reputation: 37075

Update May 2019 using RxJs v6

Found the other answers useful, and wished to offer an example for the answer offered by Arnaud about zip usage.

Here is a snippet showing the equivalence between Promise.all and the rxjs zip (note also, in rxjs6 how zip now gets imported using "rxjs" & not as an operator).

import { zip } from "rxjs";

const the_weather = new Promise(resolve => {
  setTimeout(() => {
    resolve({ temp: 29, conditions: "Sunny with Clouds" });
  }, 2000);
});

const the_tweets = new Promise(resolve => {
  setTimeout(() => {
    resolve(["I like cake", "BBQ is good too!"]);
  }, 500);
});

// Using RxJs
let source$ = zip(the_weather, the_tweets);
source$.subscribe(([weatherInfo, tweetInfo]) =>
  console.log(weatherInfo, tweetInfo)
);

// Using ES6 Promises
Promise.all([the_weather, the_tweets]).then(responses => {
  const [weatherInfo, tweetInfo] = responses;
  console.log(weatherInfo, tweetInfo);
});

The output from both are the same. Running the above gives:

{ temp: 29, conditions: 'Sunny with Clouds' } [ 'I like cake', 'BBQ is good too!' ]
{ temp: 29, conditions: 'Sunny with Clouds' } [ 'I like cake', 'BBQ is good too!' ]

Upvotes: 32

Arnaud P
Arnaud P

Reputation: 12607

On reactivex.io forkJoin actually points to Zip, which did the job for me:

let subscription = Observable.zip(obs1, obs2, ...).subscribe(...);

Upvotes: 13

kakigoori
kakigoori

Reputation: 1228

forkJoin works fine too, but I'd prefer combineLatest since you don't need to worry about it taking the last value of observables. This way, you can just get updated whenever any of them emit a new value too (e.g. you fetch on an interval or something).

Upvotes: 14

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