prebuild.mjs

// the .mjs file extention is only there so we can use nicer imports. The code is still normal JavaScript

import {exec} from 'child_process';
import {promisify} from 'util';
import {writeFile} from 'fs/promises';

// attempt to find an existing deploy for the given branch
const getDeployUrl = async (branch) => {
  const {stdout} = await promisify(exec)(`firebase hosting:channel:list --json`);
  const {result} = JSON.parse(stdout);

  return result.channels.find(deploy => deploy.name.split('/').pop() === branch)?.url;
};

// create a new hosting preview channel without deploying and return its' url
const generateBranchDeployUrl = async (branch) => {
  const {stdout} = await promisify(exec)(`firebase hosting:channel:create ${branch} --json`);
  const {result} = JSON.parse(stdout);

  return result.url;
};

// set the preview url in the CI env so future stages can use it
const setEnv = async (url) => {
  const envs = {
    // this is the name of the variable that we'll be able to use through the rest of the CI process
    // you can add more variables here if you need them
    MY_DEPLOYED_URL: url,
  };

  const stringified = Object.entries(envs)
    .reduce((prev, [key, value]) => `${prev}${key}=${value}\n`, '');

  await writeFile('build.env', stringified);
};

// ensure that there is a preview channel ready for our deploy, and put its' url in the env
const prepare = async (branch) => {
  let url = await getDeployUrl(branch);

  if (!url) {
    url = await generateBranchDeployUrl(branch);
  }

  await setEnv(url);

  console.log(`preparation complete: ${url}`);
};

prepare(
  process.argv[2] || process.env.CI_COMMIT_REF_SLUG,
)
  .catch(e => {
    console.error(e);

    process.exit(1);
  });