[Vuejs]-Add extra field on items of <v-data-table>

You can use map to add new prop to your each item in array

let medications = [{
  name: 'abc',
  id: 'naks23kn',
  resident: 1
}]

medications.map(item => item.residentName = "Your Resident Name")
console.log(medications)

This should work

watch: {
  allMedications: {
    handler: function() {
      const medicationArray = this.allMedications;
      console.log("Created this");
      this.medications = medicationArray.map(medication => medication.residentName = this.getResidentName(medication.resident)));
    },
    immediate: true
  },
}