usePrevious#

Tracks the value a source held before its latest change — a lag-by-one view. Cross-platform — import from @sigx/use.

TypeScript 
import { signal } from 'sigx';
import { usePrevious } from '@sigx/use';

const count = signal(0);
const prev = usePrevious(count); // prev.value === undefined until the first change

count.value = 5;   // prev.value === 0
count.value = 12;  // prev.value === 5

It accepts any MaybeSignal source — a signal, a computed, or a getter — and returns a ReadSignal. Until the source changes for the first time it holds initialValue (default undefined), so the return type is ReadSignal<T | undefined>.

Values are kept by reference — objects are not proxied, so identity is preserved across the lag.

Signature#

TypeScript 
function usePrevious<T>(source: MaybeSignal<T>, initialValue?: T): ReadSignal<T | undefined>;

Platform: everywhere. The internal watcher disposes with the owning scope.

See also#

  • useCounter — pair with a counter to diff against the last value.
  • watchDebounced — react to changes with a collapsed callback.
  • Conventions — MaybeSignal inputs and ReadSignal outputs.