Uncaught ReferenceError: *** is not defined?

Question

JavaScript

When the script is run, it displays an error in the console: Uncaught ReferenceError: callbackFunc is not defined what is the problem?

<div id='feedback'></div>
<input type='button' value='Test' onclick='GetUserName();'/>

VK.init({
    apiId: 5426330
  }); 

var userId = 339190723;
  
var GetUserName = function () {
VK.api('friends.get', {'user_id' : userId}, function(data) { 
  for (var i = 0, l = data.response.length; i < l; i++) {
        var script = document.createElement('SCRIPT');
    script.src = "https://api.vk.com/method/friends.get?user_id="+ data.response[i] +"&v=5.50&callback=callbackFunc";
    document.getElementsByTagName("head")[0].appendChild(script);
      function callbackFunc(result) { 
    if (result.response.items.indexOf(userId) != '-1') {
    document.getElementById('feedback').innerHTML = data.response[i];
        }
      }
    }
  });
};

Answer 1

The callback function is not defined, obviously.
It's hard to say exactly from the input data.
UPD.
Try moving the definition of the callbackFunc function higher, since at the time of connecting the script https://api.vk.com/method/friends.get?user_id=...&... this function does not yet exist.