AJAX - The Complete Reference

84 Part I: Core Ideas

conforming or coming close and the others not doing so. Figure 3-3 shows the results of

testing the common browsers at this edition’s writing; the complete results can be found at

http://ajaxref.com/ch3/requestexplorerresults.php.

Very likely, this situation is going to change as browser vendors start shoring up

the details and inconsistencies when developers start really exercising XHRs. Rather

than rely on results at one point in time, run the script at http://ajaxref.com/ch3/

requestexplorerscript.html yourself. By doing so, you help keep the chart automatically

updated until these details are worked out by the browser vendors. You may also find it

useful to use a browser HTTP debugging tool or run the Request Explorer at http://ajaxref

.com/ch3/requestexplorer.php to experiment with header values.

Other HT TP Requests

While most of time, GET and POST will be used in Ajax communication, there is a richer set

of HTTP methods that can be used. For security reasons, many of these may be disabled on

your server. You may also find that some methods are not supported in your browser, but

the first request method, HEAD, should be available in just about any case.

Head Requests

The HTTP HEAD method is used to check resources. When making a HEAD request, only

the headers are returned. This may be useful to check for the existence of a file, the file’s, or

to see if it has been recently updated before committing to fetch or use the resource.

Syntactically, there isn’t much to do differently versus previous examples except setting the

method differently, as shown here:

var url = "http://ajaxref.com/ch3/headrequest.html";

var xhr = createXHR();

if (xhr)

{

xhr.open("HEAD", url, true);

xhr.onreadystatechange = function(){handleResponse(xhr);};

xhr.send(null);

}

Authorization Delta-Base If-Unmodified-Since

Content-Base Depth Max-Forwards

Content-Location Destination MIME-Version

Content-MD5 ETag Overwrite

Content-Range From Proxy-Authorization

Content-Type If-Modified-Since SOAPAction

Content-Version If-Range Timeout

TABLE 3-6 setRequestHeader Values That Should Replace Existing Values