Ajax不会超过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);
}