Is it possible to modify XMLHttpRequest data from beforeSend callback?

Question

Is it possible to modify the data sent in an Ajax request by modifying the XMLHttpRequest object in the beforeSend callback? and if so how might I do that?

Answer 1

Yes you can modify it, the signature of beforeSend is actually (in jQuery 1.4+):

beforeSend(XMLHttpRequest, settings)

even though the documentation has just beforeSend(XMLHttpRequest), you can see how it's called here, where s is the settings object:

if ( s.beforeSend && s.beforeSend.call(s.context, xhr, s) === false ) {

So, you can modify the data argument before then (note that it's already a string by this point, even if you passed in an object). An example of modifying it would look like this:

$.ajax({
  //options...
  beforeSend: function(xhr, s) {
    s.data += "&newProp=newValue";
  }
});

If it helps, the same signature applies to the .ajaxSend() global handler (which does have correct documentation showing it), like this:

$(document).ajaxSend(function(xhr, s) {
  s.data += "&newProp=newValue";
});