scan.md

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