AJAX - The Complete Reference

Chapter 2: Pre-Ajax JavaScript Communications Techniques 39

PART I

While the image cookie technique would seem to solve the need for two-way data

transmission, do note that this technique has a major weakness: if cookies are turned off, it

is going to fail.

Two-way Script Tag Communication

As you might recall, adding a <script> tag to a document can invoke a request, and it will

expect script code back as a response.

var newScript = document.createElement("script");

newScript.src = url+"?"+payload;

newScript.type = "text/javascript";

document.body.appendChild(newScript);

In this case, we indicate in the request payload that JavaScript code should be returned and

that the code generated should invoke a call to the callback function requestComplete() so

that it can consume any returned data.

/* form query string with rating,transport,callback,and response type */

var payload = "rating=" + encodeValue(rating);

payload += "&transport=script";

payload += "&response=script";

payload += "&callback=requestComplete";

On the server side, the user-passed data is read and a result calculated just as in

previous examples. However, this time returning a result is a matter of forming a function

call and outputting it, as shown here:

if ($response == "script")

{

$message .= "$callback('$rating','$votes','$average');";

print $message;

exit;

}