部署到heroku时出现应用程序错误

heroku logs --tail

heroku config:set DATABASE_URL=your_database_url

const port = process.env.PORT || 3000;
app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});

const { Pool } = require('pg');
const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
});

web: node index.js

{
  "name": "my-app",
  "version": "1.0.0",
  "scripts": {
    "start": "node index.js"
  },
  "dependencies": {
    "express": "^4.17.1",
    "pg": "^8.7.1"
  }
}

const express = require('express');
const { Pool } = require('pg');
const app = express();
const port = process.env.PORT || 3000;

const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
});

app.get('/', (req, res) => {
  res.send('Hello World!');
});

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

web: node index.js