Here is rough sample of json request which was written while i was writting a tool for getting to do items from todoist, the requester in this sample can do a ordered request, block the 2nd request before 1st requst complete.

var bIdle = true;

function RequestAPI(sAPIUrl, iProjectID, sToken, sCallbackName, sAddtionalParameter)
{
 var argus = arguments;
 if (bIdle)
 {
  bIdle = false;
  var script = document.createElement('script');
  script.type = 'text/javascript';
  script.src = sAPIUrl + '?project_id=' + iProjectID.toString() + '&token=' + sToken + '&callback=' + sCallbackName + ((sAddtionalParameter!=null)?"&" + sAddtionalParameter:"");
  console.innerHTML +=script.src + "<br />";
  script.onreadystatechange = ReleaseAPIScript;
  document.getElementsByTagName('head')[0].appendChild(script);
 }
 else
 {
  setTimeout(function ()
  {
   RequestAPI.apply(window, argus);
  }, 1000);
 }
  
}

function ReleaseAPIScript()
{
 if (this.readyState.toLowerCase() == "loaded")
 {
  var elmtScript = this;
  // Release script after 1 min
  setTimeout(function(){
   document.getElementsByTagName('head')[0].removeChild(elmtScript);
  }, 1000);
  bIdle = true;
 }
}