index.js

exports.getUsers = functions.https.onCall((data, context) => {
  return new Promise((resolve, reject) => {
    db.collection("users")
      .get()
      .then((snapshot) => {
        if(snapshot.empty){
          reject({msg: "There is no users at all"})
        }
        data = snapshot.docs.map(doc => doc.data())
        resolve(data)
      })
      .catch((err) => {
        reject(err)
      })
  })
})