Optimistic UI

Waiting 600ms for a like to register feels slow. An optimistic value shows the expected result right away and drops it when the action ends:

const [shown, setShown] = createOptimistic(() => likes());

const like = action(function* (fail) {
  setShown((n) => n + 1);
  yield api.like(fail);
  refresh(likes);
});

shown follows likes, like a writable derived signal. A write made inside an action is temporary. When the action finishes, the temporary value is dropped and shown follows likes again. If the save worked, the refreshed count matches what was already on screen. If it failed, the count drops back by itself. There's no rollback code to write.

An action returns a promise, so the caller can catch a failed save and show the error.

Try it: make the like show immediately by writing shown inside the action. Then tick Fail next save and like again: the count rises, then falls back and the error appears.