[Vuejs]-Vue JS – Using both Vue 2 and Vue 3 in a project

That’s roughly the equivalent of:

data() {
  const {errorMessages, handleInput, setFieldData} = Validator.register(this._props);
  
  return {
    errorMessages,
    handleInput,
    input           // undefined btw
  }
}

You can alternatively install the composition api in Vue 2:

npm install @vue/composition-api

import Vue from 'vue'
import VueCompositionAPI from '@vue/composition-api'

Vue.use(VueCompositionAPI)