AJAX's origins are actually really cool. It started before XmlHttpRequest. If I recall correctly, when the user took an action that required a server response, the javascript would make a 1px by 1px iframe and load it to a specific URL with the important information in the query parameters. The response would look something like this

    <html>
     <head>
      <script>
       someGlobalFunction({
        searchResults: [
         {"name": "Joseph Paul"},
         {"name": "Joe McCray"}
        ]
       });
      </script>
     </head>
    </html>

Then once the iframe was loaded, `someGlobalFunction` would handle the results and update the page.