javascript - Firebase error: Uncaught TypeError: ref.once is not a function -

i trying use firebase upload file enter file in data base @ same time. (i keep track of how many uploads there can rename file unique) uploaded, , put copy in database can not number of how many files in child folder using once() , getchildren() methods. keep getting error. doing wrong? count number error coming here:

var ref = firebase.database();                           ref.once("value")                             .then(function(snapshot) {                               var numbersamples = snapshot.child(sampletype).numchildren(); //number of children                             }); 

full code:

<script src="https://www.gstatic.com/firebasejs/3.3.2/firebase.js"></script>             <script>               // initialize firebase               var config = {                 apikey: "aizasyc9jumxovvdgcbvxecvpriactg6ffbwtgg",                 authdomain: "instasample-d8eea.firebaseapp.com",                 databaseurl: "https://instasample-d8eea.firebaseio.com",                 storagebucket: "instasample-d8eea.appspot.com",               };               firebase.initializeapp(config);                //remove error on click               function removesampleclass() {                 $("#sampletype").removeclass('sampletypeerror')                 $("#sampletype").addclass('sampletype')               }                function removefileclass() {                 $("#filebutton").removeclass('filebuttonerror')                 $("#filebutton").addclass('filebutton')               }                 // upload files , check errors               function uploadfile(){   //on go                   //get elements                   var uploader = document.getelementbyid('uploader')                   var filebutton = document.getelementbyid('filebutton')                   //var file = filebutton.target.files[0]; //get file                   var sampletype = document.getelementbyid('sampletype').options[document.getelementbyid('sampletype').selectedindex].text;  //get folder location                   var file = filebutton.files[0]; //get file                    if (sampletype == "select type" && filebutton.value == "") {  //check both selection                     $("#sampletype").addclass('sampletypeerror')                     $("#sampletype").removeclass('sampletype')                     $("#filebutton").addclass('filebuttonerror')                     $("#filebutton").removeclass('filebutton')                   }                   else if (filebutton.value == "") {                            //check file selection                     $("#filebutton").addclass('filebuttonerror')                     $("#filebutton").removeclass('filebutton')                   }                   else if (sampletype == "select type") {                       //check sample selection                    $("#sampletype").addclass('sampletypeerror')                     $("#sampletype").removeclass('sampletype')                   }                   else{    //upload if                   uploader.classlist.remove('hide');  //unhide progress bar                   //create storage reference sampletype                    var storageref = firebase.storage().ref("/" + sampletype + "/" + file.name);                     //upload file                   var task = storageref.put(file);                    //update progress bar                   task.on('state_changed',                     function progress(snapshot){                         var percentage = (snapshot.bytestransferred / snapshot.totalbytes) * 100;                         uploader.value = percentage;                     },                     function error() {                       alert('there problem uploading file')                     },                     function complete() {                       //get root reference                       const dbreftype = firebase.database().ref().child(sampletype);                        var ref = firebase.database();                           ref.once("value")                             .then(function(snapshot) {                               var numbersamples = snapshot.child(sampletype).numchildren(); // 2 ("first", "last")                             });                         var updates = {};                         updates["/" + sampletype + "/" + sampletype + "sample" + numbersamples ] = numbersamples;                         return firebase.database().ref().update(updates);                       //sync data , return children of sampletype console                       //dbreftype.on('value', snap => console.log(snap.val()));                       //create new reference file                       //var samplefileref = dbreftype.child("sample");                         //todo: list how many in list                        //add new file list                       }                   );                 }               }               </script> 

nevermind, got it. used

dbreftype.once('value', function(snapshot) { alert('count: ' + snapshot.numchildren()); });