微信网页获取openid，中跨域请求的问题

其中请求方法如下。

```function startWeChatHttpRequest(method, url, postData, callback) {
$.ajax({
contentType: "application/json; charset=utf-8",
dataType: "json",
url: url,
type: method,
data: postData,
cache: false,
async: true,
success: function(result) {
console.log('result :', result);
callback(result);
},
error: function(result) {
console.log(JSON.stringify(result));
}
});
}

startWeChatHttpRequest("GET", "https://api.weixin.qq.com/sns/oauth2/access_token?appid=" + weChatAppId + "&secret=" + weChatSecret + "&code=" + weChatCode + "&grant_type=authorization_code", null, function(result) {
console.log('result ：', result);
if(result.access_token && null != result.access_token) {
weChatOpendId = result.openid;
} else {
alert(result);
}
});

但总是会遇到跨域请求的错误，即使改成jsonp，也不知道怎么拿到数据