scan

Accumulates a value and emits the intermediate results.

Signature

scan(accumulator: (acc: R, value: T, index: number) => R, seed?: R): OperatorFunction

Marble diagram

in:    --1--2--3--|
scan+: --1--3--6--|

What it does

Works like Array.reduce, but emits the accumulator after every next. It's the perfect building block for deriving local state from a stream of events.

When to use

Counters (clicks, progress), accumulating a list of items, small state machines, a reactive equivalent of a Redux reducer: action$.pipe(scan(reducer, initialState)).

See also

reduce, map, distinctUntilChanged