The foundation that makes AJAX possible is the communication layer with the server. The most complete option for performing this communication is the JavaScript XMLHttpRequest object. If XMLHttpRequest is not suitable to you, hidden IFrames and cookies can also be used. We will examine both options later in this chapter.
This chapter introduces you to the XMLHttpRequest object, showing you how to work around its implementation differences between browsers. After that, we make some actual page requests, both in a synchronous fashion and in an asynchronous fashion. This chapter finishes with some various fallback approaches that can be used if a browser doesn't support XMLHttpRequest, including how to use IFrames and cookies as your communication channel.
