Ajax done pass value to event trigger

I am having a click event that triggers an ajax post and from that ajax post I want to wait until it is done and get the return value from the post to be accessed in the click event. What I tried is below:

$("#tblInventory").on('click', '.user_action', function() {
  //I want to access the returned data of the function here
  $.when(GetAttachments('material')).done(function(x) {
    console.log(x); // still this is undefined
  });
});

function GetAttachments(folder) {
  var formdata = {};
  var site_url = ; 
  $.post(site_url, formdata)
    .done(function(response) {//i want to get this response
      attachment_files = response;
      console.log(response)//this console is ok
      fnShowFiles(response)


      return response;//i even tried returning the value
    })

}