我已经建立了一个MERN堆栈应用程序，它有几个不同的API调用。它的工作原理与预期的本地，但它返回504（网关超时）的所有API调用时，部署在Heroku上。我在Mongo DB Atlas的数据库和配置值已添加到Heroku配置变量正确，没有打字错误。（两者MONGODB_URI和NODE_ENV =生产）我做错了什么？

• Package

"scripts": {
    "start": "if-env NODE_ENV=production && npm run start:prod || npm run start:dev",
    "start:prod": "node server.js",
    "start:dev": "concurrently \"nodemon --ignore 'client/*'\" \"npm run client\"",
    "client": "cd client && npm run start",
    "seed": "node scripts/seedDB.js",
    "install": "cd client && npm install",
    "build": "cd client && npm run build",
    "heroku-postbuild": "npm run build"
  },

• 服务器

const express = require('express');
const mongoose = require('mongoose');
const cookieSession = require('cookie-session')
const passport = require('passport');
const bodyParser = require('body-parser')
const keys = require('./config/keys');
const PORT = process.env.PORT || 5000;

require('./models');
require('./services/passport');

mongoose.connect(process.env.MONGODB_URI || "mongodb://localhost/google-book-search", {
  useNewUrlParser: true,
  useUnifiedTopology: true,
  useCreateIndex: true
});

const app = express();

app.use(bodyParser.json());
app.use(
  cookieSession({
    maxAge: 30 * 24 * 60 * 60 * 1000,
    keys: [keys.cookieKey]
  })
)

app.use(passport.initialize());
app.use(passport.session());

require('./routes/authRoutes.js')(app);
require('./routes/billingRoutes.js')(app);
require('./routes/apiRoutes.js')(app);

// Serve up static assets (usually on heroku)
if (process.env.NODE_ENV === "production") {
  app.use(express.static("client/build"));
}

// Send every request to the React app
// Define any API routes before this runs
app.get("*", function (req, res) {
  res.sendFile(path.join(__dirname, "./client/build/index.html"));
});

app.listen(PORT, function () {
  console.log(`🌎 ==> API server now on port ${PORT}!`);
});

• 配置/密钥.js

if (process.env.NODE_ENV === 'production') {
    // Production
    module.exports = require('./prod');
} else {
    // Development
    module.exports = require('./dev');
}

• 配置/生产js

const dotenv = require('dotenv');
dotenv.config();

module.exports = {
  googleClientID: process.env.GOOGLE_CLIENT_ID,
  googleClientSecret: process.env.GOOGLE_CLIENT_SECRET,
  mongoURI: process.env.MONGODB_URI,
  cookieKey: process.env.COOKIE_KEY,
  stripePublishableKey: process.env.STRIPE_PUBLISHABLE_KEY,
  stripeSecretKey: process.env.STRIPE_SECRET_KEY
}

• 配置/开发.js

const dotenv = require('dotenv');
dotenv.config();

module.exports = {
  googleClientID: process.env.GOOGLE_CLIENT_ID,
  googleClientSecret: process.env.GOOGLE_CLIENT_SECRET,
  mongoURI: process.env.MONGODB_URI,
  cookieKey: process.env.COOKIE_KEY,
  stripePublishableKey: process.env.STRIPE_PUBLISHABLE_KEY,
  stripeSecretKey: process.env.STRIPE_SECRET_KEY
}

• .环境

GOOGLE_CLIENT_ID=some key
GOOGLE_CLIENT_SECRET=some key
MONGODB_URI=some key
COOKIE_KEY=some key