Error at parsing Json string on Javascript

Question

Im having trouble parsing a JSON string into a javascript object from a server, the code I got from this site, here is the code:

function bootstrap() {
  var myObj;
  
 
   var stringg; 
  
  var traerString = function(url) {
    var xmlhttp = new XMLHttpRequest();
    var res = null;
    xmlhttp.onreadystatechange = function() {
      if (this.readyState == 4 && this.status == 200) {
        
        res = xmlhttp.responseText;
       // console.log("resultado " + res);
        
        xml2 = this.responseData;
        
 
        return res;
        
      }
    };
    xmlhttp.open("GET", url, false);
    xmlhttp.send();
  }
  
 

  var convertiraObjeto = function(string) {
    var sarasa = JSON.parse(string);
    return sarasa;
  }

 

  stringg = traerString("https://fastspeedster.herokuapp.com/api/runners");
  console.log(stringg);
  
         myObj = JSON.parse(stringg);
        
        for (var i=0;i<myObj.COLUMNS.length;i++)
{  
        var date = myObj.COLUMNS[i].runners; 
        console.log(date);
}
       


}

And here is the output:

jQuery.Deferred exception: Unexpected token u in JSON at position 0 SyntaxError: Unexpected token u in JSON at position 0 at JSON.parse (<anonymous>) at HTMLDocument.bootstrap

Uncaught SyntaxError: Unexpected token u in JSON at position 0
at JSON.parse (<anonymous>)
at HTMLDocument.bootstrap (demo.js:40)
at j (VM1318 jquery-3.1.1.min.js:2)
at k (VM1318 jquery-3.1.1.min.js:2)

I think the issue has to do with A-sync calls but im really a noob at this, all help is aprecciated