Question

I want to add data of ExtrasGroupID variable into options serialize variable (or what is other way), how can that be done?

Example below:

var ExtrasGroupID = $("#SelectExtrasGroup option:selected").val();
var options = $("#FormExtrasOptionsList").serialize();

$.post("ajax.php", options,
  function(data)  {
        console.log(data)
  });

Answer 1

Try this:

var ExtrasGroupID = $("#SelectExtrasGroup option:selected").val();
var options = $("#FormExtrasOptionsList").serialize();

// make sure you set an appropriate key for the new value
options = options + '&' + $.param({ 'select-extras-group': ExtrasGroupID });

...

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Edit: More info on $.param: http://api.jquery.com/jQuery.param/