Javascript XMLHttpRequest Puzzler：异步与同步

Question

In building a Firefox extension, I have a dialog that calls a Javascript function when the user hits the "OK" button. That function looks like this:

var acceptSubscribeHttpReq = null;
function acceptSubscribe(url) {
    acceptSubscribeHttpReq = new XMLHttpRequest();
    acceptSubscribeHttpReq.onload = httpLoadedSubscribe;
    acceptSubscribeHttpReq.onerror = httpErrorSubscribe;
    try {
        acceptSubscribeHttpReq.open("GET", url, true);
        acceptSubscribeHttpReq.send(null);
    } catch(e) {
        acceptSubscribeHttpReq.abort();
        return false;
    }
    // alert("request.status = "+acceptSubscribeHttpReq.status);
    return true;
 }

Interestingly, this works fine if I use a synchronous send (by setting the third argument of open to false), or if I uncomment the alert at the end of the function. But if I use asynchronous without uncommenting the alert, then this fails silently -- I return from this function as expected, but the onload handler never fires.

This seems to indicate that the XMLHttpRequest object is being destroyed when the function exits. Is that correct? Is that happening because this is being called from a button? And if so, what are the possible solutions? If that's not the case, any idea why this would be happening?

Here is the XUL for the dialog:

<dialog id="winMain" title="&settingWindow.title;" style="min-width:400px;"
    onload="initSubscribe();" ondialogaccept="return acceptSubscribe();"
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">

Any and all help appreciated!

Answer 1

The field status of the request only exists after the request has finished (onload / onerror). If you try to access this field before the request has been finalized, you will have an error because such a field does not exists.