scan.md

id	scan
compareWith	reduce
lesson	21
compare	scan
video	254283000
learnBackAbout	filter
learnAbout	merge
title	The RxJS scan operator and the accumulator function
layout	default
class	post
preview_image	scan/content_preview.jpg
preview_image_alt	Scanning a stream
revised	Monday, 26 Nov. 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.

Notes

• 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).