下面方法只是简单封装，用来理解原生ajax原理

调用方法

 ajax({
            method: 'post',
            url: 'user-center/app/shop/follow',
            data: {
                access_token: 12312,
                shop_id: 123123
            },
            success: function(response) {
                console.log(response);
            }
        });

封装方法

        function ajax(opt) {
            opt = opt || {};
            opt.method = opt.method.toUpperCase() || 'POST';
            opt.url = opt.url || '';
            opt.async = opt.async || true;
            opt.data = opt.data || null;
            opt.success = opt.success || function() {};
            var xmlHttp = null;
            if (XMLHttpRequest) {
                xmlHttp = new XMLHttpRequest();
            } else {
                xmlHttp = new ActiveXObject('Microsoft.XMLHTTP');
            }
            var params = [];
            for (var key in opt.data) {
                params.push(key + ':' + opt.data[key]);
            }
            var postData = params.join('&');
            console.log(postData);
            if (opt.method.toUpperCase() === 'POST') {
                xmlHttp.open(opt.method, opt.url, opt.async);
             //设置请求头
                xmlHttp.setRequestHeader('Accept', 'application/json, text/javascript, */*; q=0.01');
                xmlHttp.setRequestHeader('Content-Type', 'application/json');
                xmlHttp.setRequestHeader('Cache-Control', 'private');
                xmlHttp.send(JSON.stringify(opt.data));
            } else if (opt.method.toUpperCase() === 'GET') {
                xmlHttp.open(opt.method, opt.url + '?' + postData, opt.async);
                xmlHttp.send(null);
            }
            xmlHttp.onreadystatechange = function() {
                if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
                    opt.success(xmlHttp.responseText);
                }
            };
        }