Saved from https://stackoverflow.com/questions/44461082/how-to-configure-rewrite-rules-inside-firebase-hosting-to-route-certain-requests

firebase1

{
  "database": {
    "rules": "database.rules.json"
  },
  "hosting": {
    "public": "build/default/public",
    "rewrites": [
      {
        "source": "/fns/**",
        "function": "fns"
      },
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]
  }
}