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Why is send so often called as xhr.send(null) instead of xhr.send() ? W3 , MDN , and MSDN all state that it's optional. Furthermore, the ActiveX control doesn't seem to need the argument : 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; The practice of send(null) goes back at least as far as 2005 in Google Maps , but being minified, there's no explanation: 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) {}

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