I had a similar misunderstanding when I first saw it, but globally declaring reactive variables and passing them around isn't really what it's about. Take, for example: https://github.com/antfu/vueuse/blob/master/packages/core/us.... Any component that wants to use a reactive reference to mouse coordinates can `const { x, y } = useMouse()` in `setup`, but `x` and `y` will refer to different objects in each of those components (since the `ref()` is instantiated inside the call to `useMouse()`). The functionality is what's shared between components, not the state/references.

That said, if you want to use the composition api to share state you can, and you can pretty easily set up some structure to restrict mutability:

    // useStore.ts
    import { reactive, readonly } from 'vue'

    export const useStore = () => {
      const state = reactive({ count: 0 })
      const increment = (val = 1) => state.count += val

      return {
        state: readonly(state), // Readonly so state can't be mutated directly...
        increment, // ...only through the mutations provided, like Vuex
      }
    }

    // store.ts
    import { useStore } from './useStore'
    export const store = useStore() // Now components can share this instance

    // MyCounter.vue
    import { store } from './store'

    export default {
      setup: () => ({
        count: store.state.count,
        onClick: () => store.increment(5),
      }),
    }

Or you can just keep using Vuex for sharing state and use the composition API for sharing self-contained bits of functionality. Or, if nothing else, using the `setup` method instead of the Options API just gives you much more control over how you organize your own code within a component.