switchMap()

Maps each outer value to an inner Store, subscribes to the latest inner, and unsubscribes from the previous. Reactive only: inner stores are created when the outer emits, not from fn(outer.get()) at build time.

Signature

function switchMap<A, B>(fn: (value: A) => Store<B>): StoreOperator<A, B | undefined>
function switchMap<A, B>(
	fn: (value: A) => Store<B>,
	opts: { initial: B },
): StoreOperator<A, B>

Parameters

Parameter	Type	Description
fn	(value: A) => Store<B>	Factory for the inner store from each outer value.
opts	{ initial?: B }	Pass { initial: B } to narrow the output type and seed get() before the first inner value.

Returns

StoreOperator<A, B | undefined> (or B when initial is set).

Basic Usage

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

const outer = state('a');
const out = pipe(
  outer,
  switchMap((x) => producer<string>(({ emit }) => { emit(x + '!'); })),
);

Options / Behavior Details

• Tier 2: Each switch starts a new reactive cycle.
• Streaming: Until first outer emission, output may be undefined unless initial is provided.

See Also

• concatMap — queue inner subscriptions
• exhaustMap — ignore while active
• flat — flatten all inner sources