Alternate Ajax Techniques, Part 1 | WebReference

Alternate Ajax Techniques, Part 1

Alternate Ajax Techniques, Part 1

By Nicholas C. Zakas.

By now, nearly everyone who works in web development has heard of the term Ajax, which is simply a term to describe client-server communication achieved without reloading the current page. Most articles on Ajax have focused on using XMLHttp as the means to achieving such communication, but Ajax techniques aren't limited to just XMLHttp. There are several other methods; we'll explore some of the more common ones in this series of articles.

Dynamic Script Loading

The first alternate Ajax technique is dynamic script loading. The concept is simple: create a new <script/> element and assign a JavaScript file to its src attribute to load JavaScript that isn't initially written into the page. The beginnings of this technique could be seen way back when Internet Explorer 4.0 and Netscape Navigator 4.0 ruled the web browser market. At that time, developers learned that they could use the document.write() method to write out a <script/> tag. The caveat was that this had to be done before the page was completely loaded. With the advent of the DOM, the concept could be taken to a completely new level.

The Technique

The basic technique behind dynamic script loading is easy, all you need to do is create a <script/> element using the DOM createElement() method and add it to the page:

var oScript = document.createElement("script");
oScript.src = "/path/to/my.js";
document.body.appendChild(oScript);

Downloading doesn't begin until the new <script/> element is actually added to the page, so it's important not to forget this step. (This is the opposite of dynamically creating an <img/> element, which automatically begins downloading once the src attribute is assigned.)

Once the download is complete, the browser interprets the JavaScript code contained within. Now the problem becomes a timing issue: how do you know when the code has finished being loaded and interpreted? Unlike the <img/> element, the <script/> element doesn't have an onload event handler, so you can't rely on the browser to tell you when the script is complete. Instead, you'll need to have a callback function that is the executed at the very end of the source file.

Example 1

Here's a simple example to illustrate dynamic script loading. The page in this example contains a single button which, when clicked, loads a string ("Hello world!") from an external JavaScript file. This string is passed to a callback function (named callback()), which displays it in an alert. The HTML for this page is as follows:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title>Example 1</title>
    <script type="text/javascript">//<![CDATA[
        function makeRequest() {
          var oScript = document.createElement("script");
          oScript.src = "example1.js";
          document.body.appendChild(oScript);
        }

        function callback(sText) {
          alert("Loaded from file: " + sText);
        }
    //]]>
    </script>
  </head>
  <body>
    <input type="button" value="Click Me" onclick="makeRequest()" />
  </body>
</html>

The JavaScript file example1.js contains a single line:

callback("Hello world!");

When the button is clicked, the makeRequest() function is called, initiating the dynamic script loading. Since the newly loaded script is in context of the page, it can access and call the callback() function, which can do use the returned value as it pleases. This example works in any DOM-compliant browsers (Internet Explorer 5.0+, Safari, Firefox, and Opera 7.0+), try it for yourself or download the examples.

More Complex Communication

Sometimes you'll want to load a static JavaScript file from the server, as in the previous example, but sometimes you'll want to return data based on some sort of information. This introduces a level of complexity to dynamic script loading beyond the previous example.

First, you need a way to pass data to the server. This can be accomplished by attaching query string arguments to the JavaScript file URL. Of course, JavaScript files can't access query string information about themselves, so you'll need to use some sort of server-side logic to handle the request and output the correct JavaScript. Here's a function to help with the process:

function makeRequest(sUrl, oParams) {
  for (sName in oParams) {
    if (sUrl.indexOf("?") > -1) {
      sUrl += "&";
    } else {
      sUrl += "?";
    }
    sUrl += encodeURIComponent(sName) + "=" + encodeURIComponent(oParams[sName]);
  }

  var oScript = document.createElement("script");
  oScript.src = sUrl;
  document.body.appendChild(oScript);
}

This function expects to be passed a URL for a JavaScript file and an object containing query string arguments. The query string is constructed inside of the function by iterating over the properties of this object. Then, the familiar dynamic script loading technique is used. This function can be called as follows:

var oParams = {
  "param1": "value1",
  "param2": "value2"
};
makeRequest("/path/to/myjs.php", oParams)

Created: March 27, 2003
Revised: February 10, 2006

URL: http://webreference.com/programming/ajax_tech/1