filter()

Forwards upstream values only when predicate returns true; otherwise holds the last passing value. Returns a StoreOperator for use with pipe().

Signature

function filter<A>(
	predicate: (value: A) => boolean,
	opts?: StoreOptions,
): StoreOperator<A, A | undefined>

Parameters

Parameter	Type	Description
predicate	(value: A) => boolean	If false, downstream gets RESOLVED (no new DATA) when the held value is unchanged.
opts	StoreOptions	Optional name and equals for memoization.

StoreOptions

Property	Type	Default	Description
name	string	undefined	Debug name for Inspector.
equals	(a: A, b: A) => boolean	undefined	Push-phase dedup when the filtered value repeats.

Returns

StoreOperator<A, A | undefined> — get() re-evaluates the predicate against the current input when disconnected.

Basic Usage

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

const n = state(0);
const evens = pipe(n, filter((x) => x % 2 === 0));
n.set(2);
evens.get(); // 2

Options / Behavior Details

• Tier 1: Participates in diamond resolution; forwards STATE from upstream.

See Also

• pipe
• map