XMLHttpRequest 整理

var xhr;
function loadXMLDoc(url) {
    xhr = null;
    if (window.XMLHttpRequest) {
        xhr = new XMLHttpRequest();
    } else if (window.ActiveXObject) {
        xhr = new ActiveXObject("Microsoft.XMLHTTP");
    }
    if (xhr != null) {
        xhr.onreadystatechange = stateChange;
        xhr.open("GET", url, true);
        xhr.send(null);
    } else {
        alert("Your browser does not support XMLHTTP.");
    }
}
function stateChange() {
    if (xhr.readyState == 4) { // 4 = "loaded"
        if (xhr.status == 200) { // 200 = OK
            // ...our code here...
        } else {
            alert("Problem retrieving XML data");
        }
    }
}

/**
 * EasyAjax
 * 简单封装 xhr 对象
 * 提供 get，post 方法
 *  ajax.get(url, params, success, error);
 */
;(function(w) {
	'use strict';
    var uid = 0,
        cache = {};
    function EasyAjax(opt) {
        this.url = opt.url;
        this.method = opt.method || 'GET';
        this.data = opt.data || '';
        this.onsuccess = typeof opt.onsuccess === 'function' ? opt.onsuccess : null;
        this.onerror = typeof opt.onerror === 'function' ? opt.onerror : null;
        this.async = typeof opt.async !== 'undefined' ? !!opt.async : true;
        this.timeout = opt.timeout || 0;
        this.xhr = null;
        this._ajax();
        this.uid = null;
    }
    EasyAjax.prototype = {
        constructor: EasyAjax,
        _ajax: function() {
            var xhr = this.xhr = this._getXHR(),
                that = this,
                data = null;
            if (xhr) {
                xhr.timeout = this.timeout;
                xhr.onreadystatechange = function() {
                    that._onreadystatuschange.call(that);
                };
                xhr.ontimeout = function() {
                    that._ontimeout.call(that);
                };
                if (this.method === 'POST') {
                    xhr.open(this.method, this.url, this.async);
                    xhr.setRequestHeader('Content-type', 'application/x-www-form-urlencoded;');
                    data = json2params(this.data);
                } else {
                    xhr.open(this.method, parseURL(this.url, this.data), this.async);
                }
                xhr.send(data);
            }
        },
        _getXHR: function() {
            var xhr = null;
            if (window.XMLHttpRequest) {
                xhr = new XMLHttpRequest();
            } else if (window.ActiveXObject) {
                xhr = new ActiveXObject('Microsoft.XMLHTTP');
            }
            return xhr;
        },
        _onreadystatuschange: function() {
            var xhr = this.xhr;
            if (xhr.readyState === 4) {
                if (xhr.status >= 200 && xhr.status < 300) {
                    this.onsuccess && this.onsuccess(xhr.responseText, xhr.statusText);
                } else {
                    this.onerror && this.onerror(xhr.responseText, xhr.statusText);
                }
                this._destory();
            }
        },
        _ontimeout: function() {
            console.error('ajax timeout: ' + this.timeout);
            // TODO
        },
        _destory: function() {
            cache[this.uid] = null;
        },
        abort: function() {
            return this.xhr.abort();
        }
    };
    function parseURL(url, json) {
        var params = json2params(json);
        return params.length > 0 ? (url + (url.indexOf('?') > -1 ? '&' : '?') + json2params(json)) : url;
    }
    function json2params(json) {
        if (typeof json === 'string') {
            return json;
        }
        var str = '';
        for (var i in json) {
            if (json.hasOwnProperty(i) && json[i] != null) {
                str += i + '=' + json[i] + '&';
            }
        }
        if (str.lastIndexOf('&') > -1) {
            str = str.slice(0, -1);
        }
        return str;
    }
    function init() {
        var obj = {},
            method = ['get', 'post'];
        for (var i = 0; i < method.length; i++) {
            obj[method[i]] = factory(method[i]);
        }
        return obj;
    }
    function factory(m) {
        m = m.toUpperCase();
        return function(url, data, onsuccess, onerror) {
            var ajax = null;
            if (typeof url === 'string') {
                ajax = new EasyAjax({
                    url: url,
                    data: data,
                    method: m,
                    onsuccess: onsuccess,
                    onerror: onerror
                });
            } else if (typeof url === 'object') {
                url.method = m;
                ajax = new EasyAjax(url);
            }
            ajax.uid = uid++;
            cache[ajax.uid] = ajax;
            return ajax;
        }
    }
    w.ajax = init();
})(window);

// 示例
ajax.get('http://uc.cn', {
    a:1,
    b:2
}, function(data, status) {
    render(JSON.parse(data));
}, function(data, status) {
    console.error(status);
});