concat

signature: concat(observables: ...*): Observable

Subscribe to observables in order as previous completes

πŸ’‘ You can think of concat like a line at a ATM, the next transaction (subscription) cannot start until the previous completes!

πŸ’‘ If throughput, not order, is a primary concern, try merge instead!

​​Ultimate RxJS​​

Examples

Example 1: Basic concat usage with three observables

( StackBlitz )

// RxJS v6+
import { of, concat } from 'rxjs';
​
concat(
of(1, 2, 3),
// subscribed after first completes
of(4, 5, 6),
// subscribed after second completes
of(7, 8, 9)
)
// log: 1, 2, 3, 4, 5, 6, 7, 8, 9
.subscribe(console.log);

Example 2: Display message using concat with delayed observables

( StackBlitz )

Example 2
// RxJS v6+
import { concat, empty } from 'rxjs';
import { delay, startWith } from 'rxjs/operators';
​
// elems
const userMessage = document.getElementById('message');
// helper
const delayedMessage = (message, delayedTime = 1000) => {
return empty().pipe(startWith(message), delay(delayedTime));
};
​
concat(
delayedMessage('Get Ready!'),
delayedMessage(3),
delayedMessage(2),
delayedMessage(1),
delayedMessage('Go!'),
delayedMessage('', 2000)
).subscribe((message: any) => (userMessage.innerHTML = message));

Example 3: (Warning!) concat with source that does not complete

( StackBlitz )

// RxJS v6+
import { interval, of, concat } from 'rxjs';
​
// when source never completes, any subsequent observables never run
concat(interval(1000), of('This', 'Never', 'Runs'))
// log: 1,2,3,4.....
.subscribe(console.log);

Additional Resources

πŸ“ Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/concat.ts​