跨域请求触发浏览器复杂请求

window.ajax = (function () {
    /**
     * 实例化并返回ajax对象
     * @return {[type]} [description]
     */
    var getXHR = function () {
        var xhr = null;
        if (typeof XMLHttpRequest != "undefined") {
            xhr = new XMLHttpRequest();
        }
        else if (typeof ActiveObject != "undefined") {
            var versions = [
                "MSXML2.XMLHttp.6.0",
                "MSXML2.XMLHttp.3.0",
                "MSXML2.XMLHttp",
            ];
            for (var i = 0; i < versions.length; i++) {
                try {
                    xhr = new ActiveObject(versions[i]);
                }
                catch (e) {
                    // 
                }
            }
        }
        else {
            throw new Error('浏览器不支持ajax');
        }
        return xhr;
    }
    /**
     * 设置发送请求是需要的参数
     * @param  {[type]} options [description]
     * @return {[type]}         [description]
     */
    var settings = function (options) {
        var opt = {};
        // method
        opt.method = typeof options.method == "undefined" ? "GET" : options.method.toUpperCase();
        if (!isAllowMethod(opt.method)) {
            opt.method = "GET";
        }

        // gateway 
        opt.url = options.url;
        if (typeof opt.url == "undefined") {
            throw new Error("请填写请求的地址");
        }

        // data
        opt.data = typeof options.data == "undefined" ? null : parseParams(options.data);
        if (opt.method == "GET" && opt.data != null) {
            opt.url = '?' + opt.data;
        }

        return opt;
    }
    /**
     * 检查是否是允许的http请求方式
     * @param  {[type]}  method [description]
     * @return {Boolean}        [description]
     */
    var isAllowMethod = function (method) {
        var allows = ["GET", "POST", "PUT", "HEAD", "OPTIONS", "DELETE", "TRACE", "CONNECT"];
        for (var i = 0; i < allows.length; i++) {
            if (allows[i] == method) {
                return true;
            }
        }
        return false;
    }
    /**
     * 处理请求是后的参数
     * @param  {[type]} data [description]
     * @return {[type]}      [description]
     */
    var parseParams = function (data) {
        var params = '';
        for (var key in data) {
            params += key + '=' + data[key] + '&';
        }
        return params.substring(0, params.length - 1);
    }
    /**
     * 设置请求的头信息
     * @param {[type]} xhr     [description]
     * @param {[type]} headers [description]
     */
    var setHeaders = function (xhr, headers) {
        if (typeof headers != "undefined") {
            for (var key in headers) {
                xhr.setRequestHeader(key, headers[key]);
            }
        }
    }
    /**
     * 对外可访问方法
     */
    return {
        run : function (options) {
            var xhr = getXHR();
            var opts = settings(options);
            // 打开链接
            xhr.open(opts.method, opts.url);
            // 设置请求头大专栏  跨域请求触发浏览器复杂请求span>
            setHeaders(xhr, options.headers);
            // 发送请求
            xhr.send(opts.data);
            // 设置响应成功后的处理
            xhr.onreadystatechange = function () {
                if (xhr.readyState == 4) {
                    if (xhr.status == 200) {
                        // console.log(xhr.responseText);
                        options.success(xhr.responseText);
                    }
                }
            }
        }
    }
})();

// 使用举例
ajax.run({
    url : 'data.txt',
    method : 'post',
    data : {
        "name" : "fantasy",
        "age" : 12
    },
    headers : {
        "Content-Type" : "application/json"
    },
    success : function (data) {
        console.log(data);
    }
});