❚ reduce takes the same three arguments as ❚ scan:

• an input stream
• an accumulator function (e.g. prepend a character to a string)
• an initial value called seed (e.g. !)

Learn more about scan ➜

Notes

As you can see, while ❚ scan returns a new stream of progressively accumulated values, ❚ reduce returns a new stream of, at most, one value. At most? If the input stream never completes, ❚ reduce will never emit any value on the output stream.

Ultimately, ❚ reduce is equivalent to ❚ scan chained with ❚ takeLast(1).

See also

accumulator

The JavaScript pipeline operator proposal

Launchpad for RxJS