问为什么我们要将null传递给XMLHttpRequest.send？

EN

xhr.send(null)

xhr.send()

hr=pIXMLHTTPRequest.CreateInstance("Msxml2.XMLHTTP.6.0");
SUCCEEDED(hr) ? 0 : throw hr;

hr=pIXMLHTTPRequest->open("GET", "http://localhost/books.xml ", false);
SUCCEEDED(hr) ? 0 : throw hr;

hr=pIXMLHTTPRequest->send(); // <-- this line
SUCCEEDED(hr) ? 0 : throw hr;

Y.asynchronousTransform = function (qc, vb, kc, Nc, Ba) {
    if (m.type == 3) return;
    var cc = Y.getCached(kc);
    if (cc) {
        cc.transformToHTML(qc, vb);
        if (Nc) Nc();
        return
    }
    var yc = qa.create(Ba);
    var sa = Xd.create();
    nd('<a href="' + kc.xmlEscape() + '">' + kc.xmlEscape() + "</a>", 0);
    sa.open("GET", kc, true);
    sa.onreadystatechange = function () {
        if (sa.readyState == 4) {
            if (yc.isValid()) {
                try {
                    var Db = sa.responseXML;
                    var cc = Y.create(Db);
                    Y.cache(kc, cc);
                    cc.transformToHTML(qc, vb);
                    if (Nc) Nc()
                } catch (b) {}
            }
        }
    };
    sa.send(null)
}