Execute an action before saving a form but after the form validation [Sharepoint]

Let’s say you want to do an asynchronous request just before saving a Sharepoint form, but after Sharepoint verified all the fields in your form (for mandatory ones, or for format, …). Here is my solution to do it (using jQuery.Deferred):

// this function will do our asynchronous check
function beforeSaving() {
  var deferred=jQuery.Deferred();
  setTimeout(function() {
    alert("Test Complete!")
    deferred.reject(); // if the test failed
    // or deferred.resolve() to valid the test
  }, 2000)
  return deferred;
}
// force PostBackRequired to true in the context, otherwise it won't work if you open it into a modal
WPQ2FormCtx.PostBackRequired=true;
// we override SPClientForms.ClientFormManager.SubmitClientForm
SPClientForms.ClientFormManager.SubmitClientForm=function(b){
  var a=SPClientForms.ClientFormManager.GetClientForm(b);
  var res = (a!=null&&a.SubmitClientForm());
  // if the form is not valid, then res===true
  if (res === true) return true;
  else {
    // at this stage, all fields' value have been saved into a hidden input
    // e.g. document.querySelector('input[type="hidden"][id$="Field_x0020_Name"]')
    // all these saved values will be sent back to the server when "WebForm_DoPostBackWithOptions" is executed

    // if the form is valid we now want to do our asynchronous check
    beforeSaving().done(function() {
      // our test is valid too so we can send the form to the server
      WebForm_DoPostBackWithOptions(new WebForm_PostBackOptions($get(WPQ2FormCtx.SubmitButtonID).name, "", true, "", "", false, true))
    }).fail(function() {
      // if it fails we just unblock the save button
      $get(WPQ2FormCtx.SubmitButtonID).disabled=false;
    })
  }
  // to make sure the form won't be submitted yet
  return true;
};