subscribeOn

Asynchronously subscribes Observers to this Observable 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.

Example

Given the following code:

import { of, merge } from 'rxjs';

const a = of(1, 2, 3);
const b = of(4, 5, 6);

merge(a, b).subscribe(console.log);

// Outputs
// 1
// 2
// 3
// 4
// 5
// 6

Both Observable a and b will emit their values directly and synchronously once they are subscribed to.

If we instead use the subscribeOn operator declaring that we want to use the asyncScheduler for values emitted by Observable a:

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

const a = of(1, 2, 3).pipe(subscribeOn(asyncScheduler));
const b = of(4, 5, 6);

merge(a, b).subscribe(console.log);

// Outputs
// 4
// 5
// 6
// 1
// 2
// 3

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 asyncScheduler for that specific Observable.