Vue3 | Pinia - Watching storeToRefs in composable function does not work

I'm trying to understand the purpose of composables. I have a simple composable like this and was trying to watch state from a Pinia store where the watch does not trigger:

import { ref, watch, computed } from "vue";
import { storeToRefs } from "pinia";
import useFlightsStore from "src/pinia/flights.js";
import usePassengersStore from "src/pinia/passengers.js";

export function useFlight() {
    const route = useRoute();

    const modalStore = useModalStore();
    const flightsStore = useFlightsStore();
    const { selection } = storeToRefs(flightsStore);

    const passengersStore = usePassengersStore();
    const { passengers, adults, children, infants } =
        storeToRefs(passengersStore);

    watch([adults, children, infants], (val) => console.log('value changes', val))

Where as the same thing in a Vue component works as expected.

So we cannot watch values inside composables?

Solution

I think you can watch values inside composables.

But, to watch a pinia state it has to be inside an arrow function:

watch(() => somePiniaState, (n) => console.log(n, " value changed"));

It's like watching a reactive object.

I believe this should be documented better. In Pinia documentation we can read how to watch the whole store or how to subscribe to a store but not how to watch a single state property inside a component or composable.

Also, the docs are somewhat shy in explaining that you can watch a property inside a store using setup() way of describing a store.

More on this here: https://github.com/vuejs/pinia/discussions/794#discussioncomment-1643242

This error also silently fails (or does not execute), which is not helpful...