promise

const create = (userUID, customerObj) => {
    return new Promise((resolve, reject) => {
        const ref = database.ref('user_data').child(userUID).child('customers').push()
        const key = ref.key;
        ref.set(customerObj)
        .then(() => {
            resolve(key)
        })
        .catch(e => {
            reject(e)
        })
    })
}