AJAX - The Complete Reference

Chapter 3: XMLHttpRequest Object 97

PART I

Response Headers

XHRs have two methods to read response headers: getResponseHeader(headername)

and getAllResponseHeaders(). As soon as the XHR has reached readyState 3, it

should be possible to look at the response headers that have been returned by the server.

Here are two simple examples:

xhr.getResponseHeader("Content-Length"); // fetches a single header

xhr.getAllResponseHeaders(); // fetches all the headers

Some possible values are shown next:

Both methods return strings, but note that in the case of multiple headers, the results will

contain \n for newlines.

If you plan on placing the headers in an XHTML page, you will have to convert the \n to

break tags or use some other preformatting mechanism to output them nicely to the screen.

var allHeaders = xhr.getAllResponseHeaders();

allHeaders = allHeaders.replace(/\n/g, "<br/>");

Looking at edge cases, there are only minor variations in browsers. For example,

attempting to fetch a header that does not exist with getResponseHeader(), may result in

a slight difference in what is returned. Firefox returns null, while IE returns nothing. Given

the loose typing system of JavaScript, this difference likely won’t be noted. Both browsers

agree what to do when you attempt to invoke these methods before headers are available:

throw a JavaScript error.

Controlling Requests

The XMLHttpRequest object has fairly limited ability to control requests once they’re sent

outside the abort() method. This method provides the basic functionality of the stop button

in the browser and will very likely be used in your Ajax applications to address network

timeouts. For example, you might imagine that you can write a cancelRequest() function