My Firebase Functions esbuild setup

buildFunctions.ts

import { build } from 'estrella'
import fs from 'fs'
import { promisify } from 'util'
import packageJSON from '../package.json'
import listFunctions from './_lib/listFunctions'

const writeFile = promisify(fs.writeFile)
const external = Object.keys(packageJSON.dependencies)
const appEnv = process.env.APP_ENV
const rootPath = process.cwd()

listFunctions().then((fns) =>
  Promise.all([
    build({
      cwd: rootPath,
      entry: `app/server/functions/_lib/init/index.ts`,
      outfile: `build/${appEnv}/functions/init.js`,
      bundle: true,
      platform: 'node',
      target: 'node9',
      sourcemap: true,
      clear: false,
      tslint: false,
      external,
    }),

    Promise.all(
      fns.map((fn) =>
        build({
          cwd: rootPath,
          entry: `app/server/functions/${fn}/index.ts`,
          outfile: `build/${appEnv}/functions/${fn}.js`,
          bundle: true,
          platform: 'node',
          target: 'node10',
          sourcemap: true,
          clear: false,
          tslint: false,
          external,
        })
      )
    ),

    writeFile(
      `build/${appEnv}/functions/index.js`,
      [
        "require('./init')",
        'function shouldRequire(fn) { return process.env.FUNCTION_TARGET === undefined || process.env.FUNCTION_TARGET === fn }',
      ]
        .concat(
          fns.map(
            (fn) =>
              `if (shouldRequire('${fn}')) module.exports.${fn} = require('./${fn}').${fn}`
          )
        )
        .join('\n\n')
    ),
  ])
)

listFunctions.ts

import fs from 'fs'
import { promisify } from 'util'

const readDir = promisify(fs.readdir)
const ignore = ['_lib', 'index.ts']
const pickedFns = process.env.FUNCTION

export default async function listFunctions() {
  if (pickedFns) return pickedFns.split(',')
  const fns = await readDir('app/server/functions')
  return fns.filter((fn) => !ignore.includes(fn))
}