Cross-browser AJAX(pure JS)

cross_browser_ajax.js

function httpRequest(url, method, data, success_callback, failure_callback) {
    var xhr;

    var data2 = [];
    if (typeof data == 'object') {
        for(var index in data) {
            if (data.hasOwnProperty(index)) {
                data2[data2.length] =index+'='+data[index];
            }
        }
        data = data2.join("&");
    }
    if(typeof XMLHttpRequest !== 'undefined') {
        xhr = new XMLHttpRequest();
    } else {
        var versions = ["MSXML2.XmlHttp.5.0",
            "MSXML2.XmlHttp.4.0",
            "MSXML2.XmlHttp.3.0",
            "MSXML2.XmlHttp.2.0",
            "Microsoft.XmlHttp"]

        for(var i = 0, len = versions.length; i < len; i++) {
            try {
                xhr = new ActiveXObject(versions[i]);
                break;
            }
            catch(e){}
        } // end for
    }

    var ua = navigator.userAgent.toLowerCase()
    if (ua.indexOf("msie") != -1 && ua.indexOf("opera") == -1 && ua.indexOf("webtv") == -1) {
        xhr.onreadystatechange = ensureReadiness;
    } else {
        xhr.onload = function (e) {
            if (this.status == 200) {
                success_callback(xhr);
            } else {
                failure_callback(xhr);
            }
        };
    }

    function ensureReadiness() {
        if(xhr.readyState < 4) {
            return;
        }

        if(xhr.status !== 200) {
            return;
        }

        // all is well
        if(xhr.readyState === 4) {
            if(xhr.status !== 200) {
                failure_callback(xhr);
            } else {
                success_callback(xhr);
            }
        }

    }

    if (method == 'post' || method == 'POST') {
        method = 'POST';

    } else {
        method = 'GET';
    }

    xhr.open(method, url, true);

    if (method == 'POST') {
        xhr.setRequestHeader("Content-type","application/x-www-form-urlencoded");
    }

    xhr.send(data);
}

it's super @Danilovonline.

why ensureReadiness?

xhr.onreadystatechange = function() {
      if (this.readyState === 4) {
        if (this.status >= 200 && this.status < 400) {
          // Success!
          success_callback(xhr)
        } else {
          // Error :(
          failure_callback(xhr)
        }
      }
    };