第一种方式：
前端

 $.ajaxSetup({
    contentType: "application/json; charset=utf-8"
  }); 

  $.post('/test', JSON.stringify({
      "Email": "sfpe@163.com",
      "Password": "123456",
      "person": {
        "age": 25
      }
    }), function(data){
    debugger
  }); 

后端

console.log(util.inspect({a: req.body}));
console.log(util.inspect({a: req.body.Email})); 

结果

{ a:
   { Email: 'sfpe@163.com',
     Password: '123456',
     person: { age: 25 } } }
{ a: 'sfpe@163.com' }

第二种方式
前端

$.post('/test', {
    post:JSON.stringify({
      "Email": "sfpe@163.com",
      "Password": "123456",
      "person": {
        "age": 25
      }
    })
  }, function(data){
    debugger
  });  

后端

console.log(util.inspect({a: req.body.post}));
console.log(util.inspect({a: req.body.post.Email}));

结果

{ a: '{"Email":"sfpe@163.com","Password":"123456","person":   {"age":25}}' }
{ a: undefined }       

问题：哪种传给后端的是json格式 ? 我用第二种传了，后端说不是json的格式....用第一种，报了这种错误： Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access.

我现在都不知道是我的问题 还是后端的问题。