how can i set ID of qty in nativescript - vue.js

Question

I have button + - and i want to save the result of that butotn + -. But i have no idea id of the button/qty. First, variable in run debug android is empty. And when i insert qty:1, the result is "variables: {qty_used: 1}". Here is the code
data() {
return {
pageData: this.tempData,
list: {},
is_checked_all: false,
is_saving: false,
is_completed: false,
qty: 1
}
},
and here is the method
methods: {
toggleSelectAll() {
if (this.is_completed) return
this.is_checked_all = true
},
save(qty) {
if (this.is_saving) return
this.is_saving = true
var vm = this
this.$apollo.query({
query: gqlUseVoucher,
variables: {
id: this.pageData.id,
qty_used: qty
},
fetchPolicy: 'no-cache'
}).then((resp) => {
if (resp.room.AuthorizeVoucherUsingID) {
vm.is_saving = false
vm.$modal.close()
vm.$store.commit('setSuccess', 'Voucher breakfast berhasil divalidasi')
} else {
vm.is_saving = false
errorHandler(vm, null, 'Voucher breakfast tidak dapat digunakan')
}
}).catch((error) => {
vm.is_saving = false
console.log(error)
errorHandler(vm, error)
})
},
now, what should i do to get the ID of that button/qty. Am new in nativescript xoxo. thank you