how to get all emails from firebasse database in single array using js

Question

Here I am getting one by one array like this:

['test@test.com']
['test@test1.com']
['test@test2.com']
['test@test3.com']

How to get two arrays into single array?

 ['test@test.com','test@test1.com','test@test2.com']

Below is my code:

    var email= myFirebased.ref().child('users');
    tags.on("child_added", function(snapshot) {
        var userkeys = snapshot.key;
        var userName = snapshot.child("userName ").val();

      var myObj = {
         name: userName ,
         keys:userkeys,
       };
        var arr = Object.keys(userName ).map(function(k) {return userName [k] });
            for(var i=0;i<arr.length;i++){
              $("#username").append('<option value ="'+ arr[i]+'"> '+ arr[i] +' </option>');
           }
  });