scan()

Accumulates upstream values with a reducer and seed, emitting the accumulator after each step. Returns a StoreOperator for use with pipe().

Signature

function scan<A, B>(
	reducer: (acc: B, value: A) => B,
	seed: B,
	opts?: StoreOptions,
): StoreOperator<A, B>

Parameters

Parameter	Type	Description
reducer	(acc: B, value: A) => B	(acc, value) => nextAcc applied on each upstream DATA.
seed	B	Initial accumulator; reset when the operator reconnects.
opts	StoreOptions	Optional equals to skip emissions when the accumulator is unchanged.

StoreOptions

Property	Type	Default	Description
equals	(a: B, b: B) => boolean	undefined	Sends RESOLVED instead of duplicate DATA.
name	string	undefined	Debug name for Inspector.

Returns

StoreOperator<A, B> — stateful; get() fold-reads the current input when disconnected.

Basic Usage

import { state, pipe } from 'callbag-recharge';
import { scan } from 'callbag-recharge/extra';

const n = state(1);
const sum = pipe(n, scan((acc, x) => acc + x, 0));
n.set(2);
sum.get(); // 3

Options / Behavior Details

• Tier 1: Forwards STATE; participates in dirty/diamond semantics.

See Also

• pipe
• reduce — final accumulated value