Javascript 部署时express server中出现cors策略错误，但即使启用了cors模块，也可以在本地服务器上正常工作

下面的代码有什么问题？我在站点上部署时遇到以下cors策略错误，但在本地主机上运行良好

代码

const express = require("express");
const api = require("./routes/api.js");
const cors = require("cors");

const PORT = process.env.PORT || 3000;

const app = express();

app.use(cors());
app.use(express.json());

app.use("/api", api);
app.use(express.static(__dirname + '/index_file_directory'))

app.get("/*", (req, res) => {
  res.sendFile('/index.html')
});

app.listen(PORT, () => console.log("Listening on http://localhost:" + PORT));

错误

Access to XMLHttpRequest at 'https://mysitename.herokuapp.com/api/login' from origin 'https://mysitename.herokuapp.com' 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.

我已经在同一台服务器上部署了angular生产构建（index.html），api端点是express router端点。路由器端点链接到mongodb atlas。 我还尝试通过在“源”选项中添加我的站点名称等方式更改cors选项，但没有任何效果