Lona: memfs + unionfs proof of concept

memfs.ts

import { FSWrapper } from '@lona/compiler/lib/helpers/fs'
import { Volume, createFsFromVolume } from 'memfs'
import { Union } from 'unionfs'
import fs from 'fs'
import path from 'path'
import { Volume as VolumeType } from 'memfs/lib/volume'

// Create a filesystem wrapper.
// Reading files: First try to read from the in-memory FS, falling back to the disk if a file is missing.
// Writing files: All files are written to the in-memory FS. We can write all files to disk later.
export const createFileSystem: () => {
  volume: VolumeType
  wrapper: FSWrapper
} = () => {
  const volume = new Volume()
  const mem = createFsFromVolume(volume)
  const union = new Union()

  union.use(fs)
  union.use(mem as any) // Cast type as temp workaround for https://github.com/streamich/unionfs/issues/453

  return {
    volume,
    wrapper: {
      readFile: (filePath: string) => {
        return Promise.resolve(union.readFileSync(filePath).toString('utf8'))
      },
      writeFile: (filePath: string, data: string) => {
        mem.mkdirpSync(path.dirname(filePath))
        return volume.promises.writeFile(filePath, data)
      },
      copyDir: (dirPath: string, output?: string) => {
        mem.mkdirpSync(path.dirname(dirPath))
        return volume.promises.copyFile(dirPath, output || '')
      },
    },
  }
}