Demo: scan

scan is very similar to reduce, except that it emits the accumulated value at each step as it accumulates a new input.

scan( (acc,input) => accumulationFunction, seedValue);

const input = of(5, 15, 25, 40, 20)
        .pipe(concatMap(v => of(v).pipe(delay(1000))));

//Emits the accumulated value  on accumulation of each new value
const addUsingScan = scan(((acc,input) => acc+input), 500);

const output = input.pipe(addUsingScan).subscribe(v=>console.log(v))

Demo: scan RunReset

Demo: scan