I'm in the process of converting a project from Vue 2 options api to Vue 3 composition api. In a options api component i have a variable called componentData, and from this component i'm calling a mixin. When i call the mixin, i can access all the component's variables from inside of the mixin, here is an example:

//myMixin.js
export default {
    methods: {
        data_changed() {
            //where componentData is declared in the component
            console.log(this.componentData)
            this.componentData = [1, 2, 3]
        },
    data() {
        return {
            someVar: null
        }
    }
}

I'm having a lot of troubles doing the same with the composition api. Instead of mixins, i have composables, so say i have a component where i'm declaring a variable:

import { ref } from 'vue'
import { testData } from '../mixins/useData.js'

export default {
    setup() {
        const componentData = ref([])
        testData()
    }
}

And my composable useData.js:

export function testData() {
    console.log(componentData.value) //undefined
    componentData.value = [1, 2, 3] //will throw error
}

In this case, the composable will return undefined. How can i access the component's data without passing them as function parameters? Is that because in Vue3 composables are only supposed to return a value and they don't work the same as mixins? Is it possible to change the value of componentData from the composable?