AJAX - The Complete Reference

Chapter 3: XMLHttpRequest Object 89

PART I

these are relatively harmless, save one in Internet Explorer that will break the object’s

functionality. To demonstrate this, first use a global XHR and set the onreadystatechange

in the wrong place, in this case before the open() method. The readyState for the XHR

object will not work properly on the second and subsequent uses, as demonstrated here:

Run: 1

Run: 2

Interestingly, the various quirks of the readyState value are rarely felt in practice since

most folks are looking solely for the final 4 value. However, you’ll see later in the chapter

that it is actually possible in some browsers and situations to look at data as it is loaded as

opposed to waiting for the final readyState value to be reached.

readyState Needs Time to Change

One particularly important aspect of asynchronous communication related to the

onreadystatechange functionality is the simple fact that in browser-based JavaScript,

callback functions cannot be invoked until the script interpreter has a free moment to do

so. There is no suspend and resume aspect to JavaScript execution in the typical single-

threaded style that is implemented in Web browsers. If you make a request and then enter

into heavy calculations or other blocking activity, control will not be handed back long

enough for the interpreter to invoke the readyState value change and the callback

function will not be invoked. You can prove this to yourself by trying to run the example at

http://ajaxref.com/ch3/longprocess.html, but given the difficulty in forcing the issue you

might rather watch a movie that shows this situation in action at http://ajaxref.com/ch3/

longprocessmovie.html.