post请求封装

function post(url, param, callback) {
    var xmlhttp = new XMLHttpRequest();
    var paraStr = '';
    xmlhttp.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            callback(JSON.parse(xmlhttp.responseText));
        }
    };
    xmlhttp.open('post', url, true);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xmlhttp.setRequestHeader("If-Modified-Since", "0");
    //下面这行 请求为ajax
    xmlhttp.setRequestHeader("X-Requested-With","XMLHttpRequest");
    for (key in param) {
        paraStr += key + "=" + encodeURIComponent(param[key]) + "&"
    }
    paraStr = paraStr.substr(0, paraStr.length - 1);
    xmlhttp.send(paraStr);

}