index.erb

<script type="text/javascript">window.data = <%= @data.to_json %>;</script>

<aat-main inline-template :arr="window.data">
... other html and v-for and v-if ...
</aat-main>  
  

main.js

//this works

const main = {
    props: {
        arr: {
            default: function() {
                if (window.data) {
                    return window.data
                } else {
                    return {

                    }
                }
            },
            type: Object
        }
    },
    data() {return {}},
    methods: {},
    created() {},
    mounted() {
        console.log(window.data)
    }
}