mergeScan

signature: mergeScan(accumulator: (acc, value, index: number) => ObservableInput, seed, concurrent: number = Number.POSITIVE_INFINITY): OperatorFunction

Accumulate value over time via merged observables.

Ultimate RxJS

Examples

Example 1: Accumulate total duration mouse held down over time

( StackBlitz )

// RxJS v6+
import { fromEvent, interval } from 'rxjs';
import { mergeScan, take, takeUntil, map, scan } from 'rxjs/operators';
// reference
const durationElem = document.getElementById('duration');
// streams
const mouseDown$ = fromEvent(document, 'mousedown');
const mouseUp$ = fromEvent(document, 'mouseup');
// accumulate time mouse held down over time
mouseDown$
.pipe(
mergeScan((acc, curr) => {
return interval(1000).pipe(
scan((a, _) => ++a, 0),
map((val: any) => val + acc),
takeUntil(mouseUp$)
);
}, 0)
// output: 1s...2s...3s...4s...
)
.subscribe(val => (durationElem.innerHTML = `${val}s`));

Additional Resources

  • pluck

    📰 - Official docs

📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/mergeScan.ts