Firebase .on（）方法被多次触发

Question

I'm making a custom firebase authentification method, so far I managed to make a form that collects users informations and send them with socket.io to the server and after that to the firebase if validation is fine. This is what I got so far, but when I start inserting in the firebase, the console.log() gets called more than once meaning that firebase tries to insert more than needed:

io.on("connection", socket => {
  socket.on("register", user => {
    console.log("Register") //gets called once, this is good!
    let db = firebase.database();
    let usersRef = db.ref("users").child(user.username);

    firebase.database().ref("users").on("value", data => { //here we read from DB
      console.log('Firebase begins here!') //this gets loged multiple times
      let isValid = true; //simple var to check if validation is true/false
      for (var el in data.val()) { //here we take user informations
        if(data.val()[el]["name"]==user.username) { // check if username exists
          console.log("Username already takken!") //gets fired more time
          socket.emit("username")
          isValid = false;
          break;
        }

        if(data.val()[el]["email"]==user.email) {
          console.log("Email is already takken!") //same story
          socket.emit("email")
          isValid = false;
          break;
        }
      } //end of for in loop
      console.log(isValid);
    if(isValid) {
      usersRef.set({
            name: user.username,
            email:user.email,
            password: user.password,
            balance: 0
      })
    }

    } , errorObject => {console.log("The read failed: " + errorObject.code)})});});

The good part is that the isValid variable prevent insertion multiple times because after the first insertion isValid changes to false and it won't insert the datas. Anyway, this is not what I'm really looking for, the ideal is to use the .on() method only 1 time, not 5-6 times..

This is how my console.logs look like:

Register event!
Firebase begins here!
true
Firebase begins here!
Username already takken!
false
Register event!
Firebase begins here!
Username already takken!
false
Register event!
Firebase begins here!
Username already takken!
false
Register event!
Firebase begins here!
true
Firebase begins here!
Username already takken!
false
Firebase begins here!
Username already takken!
false
Firebase begins here!
Username already takken!
false
Firebase begins here!
Username already takken!
false

BIG UPDATE: The issue happens only when insertions starts, if there's already a username/email the same, it won't reapeat. Ty!

Answer 1

You should use ONCE method instead of ON.

Because every time socket.on("register" is called it register callback.

1st register user = 1 callback

2nd register user = 2 callback ( new callback + old callback from 1st)

3rd register user = 3 callback ( new callback + old callback from 1st + 2nd)

let's try