index.js

#!/usr/bin/env node
const puppeteer = require('puppeteer');
const fs = require('fs');
const args = process.argv.slice(2);
const host = args[0] || 'http://127.0.0.1:8080';
const indexMatches = [
  '.gz',
  '.map',
  '3rdpartylicenses.txt',
  'ngsw-manifest.json',
  'ngsw.json',
  'worker-basic.min.js',
  'ngsw-worker.js',
  'favicon',
  'index.html',
  'manifest.webmanifest',
  '.svg'
];
let results = '';
async function main(){
  const browser = await puppeteer.launch();
  const page = await browser.newPage();

  page.on('console', msg => console.log(msg.text()));

  console.log('Page: loaded');
  await page.goto(host);

  const allRequests = new Map();
  page.on('request', req => {
    allRequests.set(req.url(), req);
  });

  await page.reload({ waitUntil: 'networkidle0' });

  Array.from(allRequests.values()).forEach(req => {
    const url = req.url();

    // filter out urls that match these extensions
    for (const exlude of indexMatches) {
      if (url.indexOf(exlude) != -1) {
        return false;
      }
    }
    // if external, dont worry about it for now
    if (url.indexOf(host) === -1) return false;

    // Format the url to remove the host
    const formatted = url.replace(`${host}/`, '');
    // if it's an empty string, just ignore it
    if (!formatted) return false;

    let type = url.substr(-3) == 'css' ? 'style' : 'script';
    results += `</${formatted}>;rel=preload;as=${type},`;
  });

  await browser.close();
  updateWith(results);
};
function updateWith(result) {
  fs.readFile('firebase.json', 'utf8', function(err, data) {
    if (err) {
      return console.log(err);
    }
    let re = /("headers":\s*\[\s*{\s*"key":\s*"Link",\s*"value":\s*")(.*)("\s*}\s*\])/gm;
    if (re.exec(data)) {
      let newConfig = data.replace(re, `$1${result}$3`);
      fs.writeFile('firebase.json', newConfig, 'utf8', function(err) {
        if (err) return console.log(err);
        console.log('firebase.json updated successfully.');
      });
    } else {
      console.log("Couldn't find a valid firebase config to update.");
      return;
    }
  });
}
main();

Should host be defined somewhere?

fixed!

On a base firesbase.json file that regex method is returning null - any thoughts?

update: it looks like you need to have an empty headers group in that file for the regex to grab and then fill in with the updated value.