"Synchronous XMLHttpRequest on main thread is deprecated" error in some browsers. How to fix?

Question

JavaScript

In google chrome the code runs fine. But in firefox and opera, this code does not work - it gives an error.

function GetAddFormAndOpenModal(form, promo, code){

    if(form){
      $.post('/personal/items/add/', 
       { itemadd : form['itemadd'].value, submitform : form['submitform'].value, checkbutton : form['checkbutton'].value,
        promo : form['promo'].value, code : form['code'].value, lastchoose : form['lastchoose'].value },
       function(html){
        $("#chooseAdd div.content").html(html);	
      }, "html");
    }else{
      $.post("/personal/items/add/?promo="+promo+"&code="+code, function(html){
        OpenModalWindow2('chooseAdd');
        $("#chooseAdd div.content").html(html);	
      }, "html");
    }
  }

Help! Where is the mistake?

Answer 1

This is the jamb of the browser developers.
request.open('POST', "/personal/items/add/", true);
Since "true" is an asynchronous connection, not a synchronous one, it turns out that the browser is swearing by mistake. Personally, this happens to me when the connection to the server where request.open is being hammered is simply not available, and if the server is available, then these errors do not exist.

Answer 2

I rewrote the code to:

function GetAddFormAndOpenModal(form, promo, code){
  if(form){
    var params = 'itemadd=' + encodeURIComponent(form['itemadd'].value) 
      + '&submitform=' + encodeURIComponent(form['submitform'].value)
      + '&checkbutton=' + encodeURIComponent(form['checkbutton'].value)
      + '&promo=' + encodeURIComponent(form['promo'].value)
      + '&code=' + encodeURIComponent(form['code'].value)
      + '&lastchoose=' + encodeURIComponent(form['lastchoose'].value)
      ;
    var request = new XMLHttpRequest(); 
    request.onreadystatechange = processRequestChange;
    request.open('POST', "/personal/items/add/", true); 
    request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    request.send(params); 
  }else{
    var request = new XMLHttpRequest(); 
    request.onreadystatechange = processRequestChange;
    request.open('GET', "/personal/items/add/?promo="+promo+"&code="+code, true);
    request.send(null);
  }
}

Runs fine on chrome, but firefox still gives the error "Synchronous XMLHttpRequest on the main thread is deprecated due to its detrimental effect on the end user experience."
At the same time, I set request.open() to true - which means an asynchronous request.