function stable

Asynchronously subscribes Observers to this Observable on the specified SchedulerLike.

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



The SchedulerLike to perform subscription actions on.


Optional. Default is 0.

Type: number.


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


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.

subscribeOn marble diagram


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.

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