Asynchronously subscribes Observers to this Observable on the specified SchedulerLike.

subscribeOn<T>(scheduler: SchedulerLike, delay: number = 0): MonoTypeOperatorFunction<T>

Parameters

Returns

MonoTypeOperatorFunction<T>: The source Observable modified so that its subscriptions happen on the specified SchedulerLike. .

Description

With subscribeOn you can decide what type of scheduler a specific Observable will be using when it is subscribed to.

Schedulers control the speed and order of emissions to observers from an Observable stream.

Example

Given the following code:

import { of, merge } from 'rxjs';

const a = of(1, 2, 3, 4);
const b = of(5, 6, 7, 8, 9);
merge(a, b).subscribe(console.log);

Both Observable a and b will emit their values directly and synchronously once they are subscribed to. This will result in the output of 1 2 3 4 5 6 7 8 9.

But if we instead us the subscribeOn operator declaring that we want to use the async for values emited by Observable a:

import { of, merge, asyncScheduler } from 'rxjs';
import { subscribeOn } from 'rxjs/operators';

const a = of(1, 2, 3, 4).pipe(subscribeOn(asyncScheduler));
const b = of(5, 6, 7, 8, 9);
merge(a, b).subscribe(console.log);

The output will instead be 5 6 7 8 9 1 2 3 4. The reason for this is that Observable b emits its values directly and synchronously like before but the emissions from a are scheduled on the event loop because we are now using the async for that specific Observable.