Following the instructions in https://ionicframework.com/docs/vue/quickstart#build-your-way-with-typescript-or-javascript to convert a fresh project to javascript by removing all ts dependencies doesn’t seem to be enough for me.
I get a build warning that warning in ./node_modules/@ionic/core/dist/esm-es5/index-7a8b7a1c.js [vue-cli-service] "export '__spreadArray' was not found in 'tslib'
and a console error in the served app that index-7a8b7a1c.js?6d28:1 Uncaught (in promise) TypeError: Object(...) is not a function
which is also tslib related i reckon.