Sava Data to a variable Place [Firebase]

Question

I want to write some User Data to a variable place. But I don't know how I can tell this to Firebase. The normal save function doesn't work with a variable save location. Like in the example below I want to save the data on a variable save location.

function Writevariabledata(userId,data) 
{
    firebase.database().ref('user/testdata/' + userId).set({
    $userId: data
     });
}

Answer 1

this works for me

function Writevariabledata() {
  var name = document.getElementById("name");
  var firebaseref = firebase.database().ref("user/testdata/" + userId);
  var messagetext = {};
  messagetext.cat_name = name.value;
firebaseref.push().set(messagetext);
 window.alert('Inserted Successfully..');