两个不同端口上的两个节点服务器：尽管安装了标头定义和CORS软件包，仍然会出现CORS错误

Question

后端Api位于端口5000上，前端客户端位于端口4000上。我已经实现了其他SO posts和MSDN的解决方案，但仍然出现错误。

已安装NPM软件包：

npm install --save cors

两台服务器的app.js文件的实现方式

：

    var cors = require('cors');
    app.use(cors({
      credentials: true,
    }));


 app.use((req, res, next) => {
  res.append('Access-Control-Allow-Origin', ['*']);
  res.append('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
  res.append('Access-Control-Allow-Headers', 'Content-Type, token, Content-Length, X-Requested-With, *');
  next();
});

在客户端js中的实现：

var _HEADERS = new Headers({
    "Content-Type": "application/json",
    'Access-Control-Allow-Origin':"*",
    "Access-Control-Allow-Credentials": true
});

let policy= {
    mode: "cors",
    cache: "no-cache",
    headers: _HEADERS
};
return fetch(url,  policy) 
    .then((resp) => {    
            console.log(resp);
            // return resp.json();
    })    
    .catch(function(e) {      
       console.log(e);
    });

错误：

Access to fetch at 'http://localhost:5000/api/...}' 
from origin 'http://localhost:4000' has been blocked by CORS policy: 

Response to preflight request doesn't pass access control check: 
No 'Access-Control-Allow-Origin' header is present on the requested resource...