create-reaction.mdx

title	createReaction

function createReaction(onInvalidate: () => void): (fn: () => void) => void

Sometimes it is useful to separate tracking from re-execution. This primitive registers a side effect that is run the first time the expression wrapped by the returned tracking function is notified of a change.

const [s, set] = createSignal("start")

const track = createReaction(() => console.log("something"))

// next time s changes run the reaction
track(() => s())

set("end") // "something"

set("final") // no-op as reaction only runs on first update, need to call track again.