Using RxJS `concat` to Merge Observables Sequentially

rxjs merging obsevables how to having two observables get the second after first complete

In RxJS, you can use the concat operator to merge two observables in such a way that the second observable starts emitting items only after the first observable completes. The concat operator subscribes to each observable in sequence, waiting for each one to complete before moving on to the next.

Here's an example of how you can use concat to achieve this:

import { of, concat } from 'rxjs';
import { delay } from 'rxjs/operators';

// Create the first observable
const firstObservable = of('First Observable - 1', 'First Observable - 2').pipe(
  delay(1000) // Adding delay to simulate async operation
);

// Create the second observable
const secondObservable = of('Second Observable - 1', 'Second Observable - 2').pipe(
  delay(1000) // Adding delay to simulate async operation
);

// Use concat to merge the observables
const mergedObservable = concat(firstObservable, secondObservable);

// Subscribe to the merged observable
mergedObservable.subscribe({
  next: value => console.log(value),
  complete: () => console.log('All observables completed')
});

In this example:

1. firstObservable emits two values with a delay of 1 second.
2. secondObservable also emits two values with a delay of 1 second.
3. The concat operator is used to merge these observables. It ensures that secondObservable starts emitting values only after firstObservable has completed.
4. The subscribe method is used to listen to the merged observable and log the emitted values to the console.

When you run this code, you should see the following output in the console:

First Observable - 1
First Observable - 2
Second Observable - 1
Second Observable - 2
All observables completed

This demonstrates that the second observable starts emitting values only after the first observable has completed.