Ajax无法通过readyState 1，为什么？

我解决了分配onload事件而不是onreadystatechange的问题：

function Request(url, callback){if (window.XMLHttpRequest) { // Mozilla, Safari, ...    httpRequest = new XMLHttpRequest();} else if (window.ActiveXObject) { // IE    httpRequest = new ActiveXObject("Microsoft.XMLHTTP");} else{        return false;}var readyStateChange = function(){    console.log(httpRequest.readyState);    if (httpRequest.readyState == 4) {     callback(httpRequest.responseText);    }};if (isFirefox && firefoxVersion > 3) {    httpRequest.onload = readyStateChange;} else {    httpRequest.onreadystatechange = readyStateChange;}console.log(httpRequest, url);httpRequest.open('GET', url, true);httpRequest.send(null);}