rest/ahah

AHAH: Asychronous HTML and HTTP

AHAH is a very simple technique for dynamically updating web pages using JavaScript. It involves using XMLHTTPRequest to retrieve (X)HTML fragments which are then inserted directly into the web page, whence they can be styled using CSS.

Contributors

• David Hansson
• Kevin Marks
• Ernest Prabhakar

Relation to AJAX

AHAH is intended to be a much simpler way to do web development than AJAX: "Asynchronous JavaScript and XML." Strictly speaking, AHAH can be considered a subset of AJAX, since (X)HTML is just a special kind of XML. However, it is a subset with some very specific and useful properties:

1. The lack of custom XML schemas dramatically reduces design time
2. AHAH can trivially reuse existing HTML pages, avoiding the need for a custom web service
3. All data transport is done via browser-friend HTML, easing debugging and testing
4. The HTML is designed to be directly embedded in the page's DOM, eliminating the need for parsing
5. As HTML, designers can format it using CSS, rather than programmers having to do XSLT transforms
6. Processing is all done on the server, so the client-side programming is essentiall nil (moving opaque bits)

In fact, for any content that is destined to be viewed by the browser, it is virtually impossible to imagine any advantage to sending it as custom XML rather than structurally-correct HTML (with appropriate CSS-friendly class names, of course).

That said, many applications of AJAX are (at least in theory) targeteable at custom JavaScript code or desktop GUIs rather than mere browsers. For those cases, the advantages of HTML over custom XML are somewhat less. However, even here, it may well make sense to encode data using xoxo -- aka XHTML Property Lists -- which can be losslessly converted back and forth from standard data structures (lists and dictionaries) without the need for custom parsers.

Source Code

Unlike the various libraries (e.g., JSON, MochiKit) important for AJAX, all of AHAH is contained in a single JavaScript file (also available as ahah.js and jah.js). In fact, this is little more than the canonical XMLHttpRequest example, and is simple enough for any modern web designer to embed within their existing web pages.

Send AHAH Request

function ahah(url,target) {
   // native XMLHttpRequest object
   document.getElementById(target).innerHTML = 'sending...';
   if (window.XMLHttpRequest) {
       req = new XMLHttpRequest();
       req.onreadystatechange = function() {ahahDone(target);};
       req.open("GET", url, true);
       req.send(null);
   // IE/Windows ActiveX version
   } else if (window.ActiveXObject) {
       req = new ActiveXObject("Microsoft.XMLHTTP");
       if (req) {
           req.onreadystatechange = function() {ahahDone(target);};
           req.open("GET", url, true);
           req.send();
       }
   }
}    

Note the workaround needed for IE's ActiveX implementation. The current version hard-codes GET; there may be value in adding an extra parameter to allow POST, PUT, and DELETE.

Receive AHAH Request

function ahahDone(target) {
   // only if req is "loaded"
   if (req.readyState == 4) {
       // only if "OK"
       if (req.status == 200) {
           results = req.responseText;
           document.getElementById(target).innerHTML = results;
       } else {
           document.getElementById(target).innerHTML="ahah error:\n" +
               req.statusText;
       }
   }
}

Other than error checking, the only real work done is:

document.getElementById(target).innerHTML = results;

Everything else (e.g., CSS-styling) is merely inherited from the parent webpage.

Executing Javascript

Since the browser won't execute <script> when changing innerHTML, you may want to apply the following function call on document.getElementById(target):

var bSaf = (navigator.userAgent.indexOf('Safari') != -1);
var bMoz = (navigator.appName == 'Netscape');
function execJS(node) {
  var st = node.getElementsByTagName('script');
  for(var i=0;i<st.length; i++) {
    if (bSaf) {
      eval(st[i].innerHTML);
    }
    else if (bMoz) {
      eval(st[i].textContent);
    }
    else {
      eval(st[i].text);
    }
  }
}

History

"AHAH" as a formal technique appears to have been introduced by Kevin Marks on May 12, 2005 under the name JAH: "Just Asynchronous HTML", where it was also used in a simple example. The term "AHAH" was proposed by Ernest Prabhakar during the 2005 Web 2.0 conference, and later adopted as part of the REST-Enabled XHTML (REX) microformat for web services.

David Hansson had independently discovered the exact same concept, and in fact had already submitted an abstract about it for O'Reilly's 2006 E-Tech conference when he encountered the work done by Marks and Prabhakar. He had not however named the technique, and quickly agreed to adopt the AHAH moniker. The same concept has no doubt been independently discovered by others, but these three appear to be the first to make a sustained attempt to promote it as a formal technique.

Implementations

• Sample JavaScript code available from Ernie Prabhakar, extending work by Kevin Marks.
• There is some talk of directly supporting AHAH in Ruby on Rails using partials.