api.js

export async function test() { }

index.js

const path = require("path")
const fs = require('fs')

const defaultConfig = require("@parcel/config-default")
const { default: Parcel, createWorkerFarm } = require("@parcel/core")
const { NodeFS, MemoryFS } = require("@parcel/fs")

const workerFarm = createWorkerFarm()
const nodeFS = new NodeFS()
const memoryFS = new MemoryFS(workerFarm)

// set to memoryFS to using the memory file system as input
let inputFS = nodeFS

const BASE_PATH = '/build'
const SRC_PATH = `${BASE_PATH}/src`
const DIST_PATH = `${BASE_PATH}/dist`
const CODE_PATH = path.join(__dirname, 'src', 'api.js')

const bundler = new Parcel({
  entries: inputFS == memoryFS ? `${SRC_PATH}/api.js` : CODE_PATH,
  cache: false,
  defaultConfig: {
    ...defaultConfig,
    filePath: require.resolve("@parcel/config-default"),
  },
  defaultEngines: {
    browsers: ["last 1 Chrome version"],
  },
  targets: {
    main: {
      engines: { browsers: ["last 1 Chrome version"] },
      distDir: `${DIST_PATH}`
    }
  },
  inputFS: inputFS,
  outputFS: memoryFS,
  workerFarm,
})

fs.readFile(path.join(__dirname, 'src', 'api.js'), async (err, code) => {
  await memoryFS.mkdirp(SRC_PATH)
  await memoryFS.mkdirp(DIST_PATH)
  await memoryFS.writeFile(`${SRC_PATH}/api.js`, code)
  console.log('bunder starting')
  await bundler.run()
  console.log('bunder complete')
  const files = await memoryFS.readdir("/build/dist/")
  for (let file of files) {
    console.log(file)
  }
  await workerFarm.end()
});

package.json

{
  "name": "parcel-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "parcel": "^2.0.0-alpha.3.2"
  }
}