Javascript Express 路由在本地工作正常，但在 Heroku 上却不行

我的express应用程序在本地主机上运行良好，但在Heroku上不起作用.

the line is

app.use("/api/product", require("./routes/product"))

这是密码

const express = require("express");
const app = express();

const port = process.env.PORT || 5000;


app.get("/", (req, res) => {
    res.send("responded")
});

app.use(express.json())

app.use("/api/product", require("./routes/product"))


app.listen(port, () => {
    console.log(`Example app listening at http://localhost:${port}`);
});

product.js

const express = require("express");
const router = express.Router();

router.get("/", async (req, res) => {
    try {
        res.json({
            status: 200,
            message: "Data has been successfully fetched"
        });
    }
    catch (error) {
        console.log(error);
        return res.status(400).send("server error")
    }
})

module.exports = router;

package.json

{
  "name": "backend-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.17.3"
  }
}

文件夹 struct