I'm quite new to JS and i have a problem. I'm trying to get XMLHttpRequest.responseText to use it as variable in GTM. The url I'm sending request is an endpoint.

function httpGetAsync (url, callback) {
  var xmlHttp = new XMLHttpRequest();
  xmlHttp.open("GET", url, true);
  xmlHttp.onreadystatechange = function() {
    if(xmlHttp.readyState == 4 && xmlHttp.status == 200){
    callback(xmlHttp.responseText);
    }
  }
  xmlHttp.send();

}

httpGetAsync("https://www.example.pl/example-uid-cookie", function(response) {
    console.log(response);
});

The answear I'm getting looks like below and I would like to have the value instead of undefined: