usePrevious

usePrevious(fn) wraps fn so that on every call it receives the previous return value as its second argument — useful for transitions, deltas, or "compute from the previous state" patterns without an extra signal. Part of pota/use/selector.

The wrapper keeps the previous value in a closure variable, so it is not reactive on its own; reach for it inside a derivation or effect when you need to compare a value against the one it produced last time.

Arguments

Argument	Type	Description
fn	(next, previous) => T	Called with the current input and the value it returned on the prior call.

Returns: a function next => T that runs fn(next, previous), stores the result as the new previous, and returns it. On the first call previous is undefined.

Examples

Compute from the previous return

Wrap a reducer-style function so each call sees what it returned last time. On the first call the previous value is undefined.

import { render, signal } from 'pota'
import { usePrevious } from 'pota/use/selector'

function App() {
	const log = signal('')
	const step = usePrevious((next, prev) => {
		log.write(`was ${prev} → ${next}`)
		return next
	})

	return (
		<div>
			<p>{log.read}</p>
			<button on:click={() => step(1)}>step(1)</button>
			<button on:click={() => step(2)}>step(2)</button>
			<button on:click={() => step(5)}>step(5)</button>
		</div>
	)
}

render(App)