Firebase emulator: add build steps to hosting

Question

I'm trying to programmatically build my index.html file from a template, using a custom script/pre-deploy hook.

Is it possible to have the Firebase emulator run my predeploy script either (1) on start, or (2) on page refresh, so that I can see in the emulator what will happen on actual deploy?

My script works in actual deployment, but never fires with the emulator.

firebase.json (relevant part)

"hosting": {
  "public": "public",
  "ignore": [
    "firebase.json",
    "**/.*",
    "**/node_modules/**"
  ],
  "predeploy": "node ./preDeploy.js",
  "rewrites": [
    {
      "source": "**",
      "destination": "/index.html"
    }
  ]
},

preDeploy.js (oversimplified minimal example)

const fs = require('fs');
const testString = 'hello';
fs.writeFileSync('./preDeployTest.txt', testString);

I would expect the test file to be generated (1), ideally every time I refresh my site while running the emulator, or (2) at least when I first start the emulator -- but the script only runs on actual deploy.

To be clear, I'd be using this script to generate the file that I want Firebase hosting to use as its index.html, but I want to be able to emulate the results.

Answer 1

This may be possible with the default lifecycle scripts block:

"scripts": {
    "prepare": "node ./prepare.js"
}

There's also preprepare and a few other hooks; you may have to toy with it to find the correct one. The point is, that one isn't limited to Firebase specific hooks, but can use those defined by NodeJS.