Intercepting fs.promises methods to customize behaviour at runtime with Proxies

fs-promises-proxy.js

// Original source code: https://github.com/ErickWendel/securing-files-using-nodejs-crypto-yt
// Proxy documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

// app.js
// `promises` is now a param instead of an import
async function run(promises) {
  ...
}
  
// index.js
const CryptoHelper = require('./src/cryptoHelper')
const app = require('./src/app')

;(async () => {
  const config = {
    // aes-192
    // 24 caracteres * 8 = 192 bits
    cryptoKey: 'minha-senha-super-segura',
  }
  const cryptoHelper = await CryptoHelper.setup(config)

  const writeFileHandler = {
    apply: async function (target, that, args) {
      const [filename, data, encoding = ''] = args
      const encryptedText = await cryptoHelper.encrypt(data)

      return target(filename, encryptedText, encoding)
    },
  }

  const readFileHandler = {
    apply: async function (target, that, args) {
      const data = await target(...args)
      const decrypted = await cryptoHelper.decrypt(data)

      return decrypted
    },
  }

  const promisesHandler = {
    get: function (target, prop, receiver) {
      switch (prop) {
        case 'writeFile':
          return new Proxy(target[prop], writeFileHandler)
        case 'readFile':
          return new Proxy(target[prop], readFileHandler)
        default:
          return Reflect.get(...arguments)
      }
    },
  }

  const customFsPromises = new Proxy(require('fs').promises, promisesHandler)

  await app.run(customFsPromises)
})()