[Vuejs]-How to combine each select tag's data

You could play around with substrings to get/set the first 4 chars for year, 2 for month and 2 for days of the birthDate string.

But if it was me, I would keep the year, month and day separate in the store and combine when calling the api:

axios.post(‘http://??.???.???.???:????/api/signup’, {
…state.Userinfo,
birthDate: `${state.birthYear}${state.birthMonth}${state.birthDay}`,
});