CORS相关问题，我无法继续POST请求

Question

我在尝试使后端和前端在不同的本地主机端口（4000和3000）上相互通信（reactjs vanilla NODEjs）时遇到了一些问题

我设法让它工作，直到我想尝试添加数据到我的数据库。我可以用我的react应用程序获取数据，但我不能发布任何数据。以下是双方的代码:

前面 ： '

const sendData = async (event: any): Promise<void> => {
    console.log("hello");
    event.preventDefault();
    const response = await fetch(`${_DATA_URL_}`, {
      method: "POST",
      headers: { "Content-Type": "application/json" },
      body: JSON.stringify(employeeForm),
    });
    const data = await response.json();
    console.log(data);
  };

`后退:

const Router = (req, res) => {
  res.setHeader("Access-Control-Allow-Origin", "*");
  res.setHeader(
    "Access-Control-Allow-Headers",
    "Content-Type, Accept, Origin, Authorization"
  );
  res.setHeader(
    "Access-Control-Allow-Methods",
    "GET, POST, PUT, DELETE, PATCH, OPTIONS"
  );
  console.log("hello"); //debug
  if (req.method === "GET") {
    if (req.url === "/api/employees") {
      getEmployees(req, res);
    } else if (req.url.match(/\/api\/employees\/(\w{4}-?){3}/)) {
      const requestID = req.url.split("/")[3];

      getEmployee(req, res, requestID);
    } else {
      res.statusCode = 404;

      res.setHeader("Content-Type", "application/json");
      res.end(JSON.stringify({ message: `Error : ${req.url} not found !` }));
    }
  }
  if (req.method === "POST") {
    if (req.url === "/api/employees") {
      createEmployee(req, res);
    } else {
      res.setHeader("Content-Type", "application/json");
      res.end(JSON.stringify({ message: `Error : ${req.url} not found !` }));
    }
  }
  if (req.method === "DELETE") {
    if (req.url.match(/\/api\/employees\/(\w{4}-?){3}/)) {
      const requestID = req.url.split("/")[3];
      deleteEmployee(req, res, requestID);
    } else {
      res.statusCode = 404;

      res.setHeader("Content-Type", "application/json");
      res.end(JSON.stringify({ message: `Error : ${req.url} not found !` }));
    }
  }
  if (req.method === "PUT") {
    if (req.url.match(/\/api\/employees\/(\w{4}-?){3}/)) {
      const requestID = req.url.split("/")[3];
      updateEmployee(req, res, requestID);
    } else {
      res.statusCode = 404;

      res.setHeader("Content-Type", "application/json");
      res.end(JSON.stringify({ message: `Error : ${req.url} not found !` }));
    }
  }
};

我很确定它来自react端，因为当我使用POSTMAN时，发布数据工作正常，在本地主机上过了一段时间后，我在控制台中收到了这个错误：

也许我使用cors不正确，无论如何感谢你的未来答案

Answer 1

我想通了，

在这里（法语）和这里回答