• startWith (soon)
  • endWith (soon)
  • concat (soon)


Compare scan with reduce

Animated example
How it works


accepts 1 input stream, an accumulator, a seed (optional)

  • The seed will be used as an initial accumulation acc
  • When the input stream emits a value v:
    • v and the latest acc are given to the accumulator
    • the returned value is emitted and will be used as the next acc

returns a new stream of accumulated values

Published on Monday, 5 Feb. 2018
Revised on Monday, 8 Oct. 2018

Scan arguments

❚ scan takes three arguments:

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

The accumulator function combines two values:

  • an accumulated value called accumulation
  • and a value

As a result, the accumulator returns a new accumulation.


  • As you can see, the output stream does not start with the seed. This is how it works, for example, in RxJS.
  • In some other reactive stream libraries (like xstream and Most.js) the output stream starts with the seed.
  • To start with the seed in RxJS and such, you can chain ❚ startWith(seed) and ❚ scan(f, seed).

Cédric Soulas

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