Add this file into the root of express server to deploy on vercel

vercel.json

{
  "version": 2,
  "builds": [
    {
      "src": "app.js",
      "use": "@vercel/node"
    }
  ],
  "routes": [
    {
      "src": "/(.*)",
      "dest": "app.js",
      "methods": ["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"]
    }
  ]
}