Add type def to data object - Vue

Below is a simple Vue single file component that I Have created and I want to add type definition to its data function,

How can I add type info without using typescript ?

<template>
   <h2> {{ title }} </h2>
</template>
<script>
 export default {
 name: 'test',
 data () {
      return {
          title: 'Hello World'
       }
    }
}
</script>