node路由

router.post('/',(req,res)=>{
    console.log(req.body,1);
    res.json({name:1,age:3})
});

javascript代码

function ajax(options){
    let {url,success} = options,
        type = options.type || 'GET',
        data = options.data || null,
        contentType = options.contentType || 'application/x-www-form-urlencoded';
    let req = new XMLHttpRequest();
    req.open(type,url,true);
    if(/^POST$/i.test(type)) req.setRequestHeader('Content-Type',contentType);
    req.onreadystatechange = function(){
        console.log(req.status,req.readyState);
        if(req.readyState === 4 && req.status === 200){
            success && success(req.response);
        }
    }
    req.send(data);
}

ajax({
    url:'/aaa',
    type:'post',
    data:{"a":1,"b":2},
    success:(data)=>{
        console.log(data);
    }
});

data是 {"a":1,"b":2}
后台输出 { 'object Object': '' } 1

data是 "abc"
后台输出 { abc: '' } 1

data是

var formData = new FormData();
formData.append('a',1);

后台输出

{ '------WebKitFormBoundaryhR4KppDAZDAMRtnj\r\nContent-Disposition: form-data; name': '"a"\r\n\r\n1\r\n------WebKitFormBoundaryhR4KppDAZDAMRtnj--\r\n' } 1

这是什么问题？平常都是用的jquery的ajax传的没什么事。
后台向前台传数据可以正常接受