How to get the previous value for a solidjs signal?

Question

I want to get the previous value of a signal how?

const [route, setRoute] = createSignal(1)

createEffect(() => {
  console.log('route previous value is', route(0))
  console.log('route updated value is', route())
})

Answer 1

The callback function you provide to createEffect can take an argument than is equal to the returned value from the effect function's last call. Therefore, you can do the following:

const [route, setRoute] = createSignal(1);

createEffect((prev) => {
  const updated = route();

  console.log('route previous value is', prev);
  console.log('route updated value is', updated);

  return updated;
}, route());

Note that the second argument you pass to createEffect is the initial value for prev.

This described in the createEffect section of the Solid API docs.