在Heroku中部署MERN堆栈应用程序时出现问题

const express = require('express');
const mongoose = require('mongoose');
const path = require('path');

const app = express();
const PORT = process.env.PORT || 5000;

// 连接MongoDB
mongoose.connect(process.env.MONGODB_URI, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
}).then(() => console.log('MongoDB connected'))
  .catch(err => console.log(err));

// 设置静态文件目录
app.use(express.static(path.join(__dirname, 'client/build')));

// API路由
app.use('/api', require('./routes/api'));

// 所有其他请求重定向到React应用
app.get('*', (req, res) => {
  res.sendFile(path.join(__dirname+'/client/build/index.html'));
});

app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

{
  "name": "mern-stack-app",
  "version": "1.0.0",
  "scripts": {
    "start": "node server.js",
    "build": "cd client && npm run build",
    "heroku-postbuild": "npm run build"
  },
  "dependencies": {
    "express": "^4.17.1",
    "mongoose": "^5.12.3",
    "react": "^17.0.2",
    "react-dom": "^17.0.2",
    "react-scripts": "4.0.3"
  }
}

heroku logs --tail