wallet-core/node_modules/fs-extra/lib/fs/index.js

// This is adapted from https://github.com/normalize/mz
// Copyright (c) 2014-2016 Jonathan Ong me@jongleberry.com and Contributors
const u = require('universalify').fromCallback
const fs = require('graceful-fs')

const api = [
  'access',
  'appendFile',
  'chmod',
  'chown',
  'close',
  'fchmod',
  'fchown',
  'fdatasync',
  'fstat',
  'fsync',
  'ftruncate',
  'futimes',
  'lchown',
  'link',
  'lstat',
  'mkdir',
  'open',
  'readFile',
  'readdir',
  'readlink',
  'realpath',
  'rename',
  'rmdir',
  'stat',
  'symlink',
  'truncate',
  'unlink',
  'utimes',
  'writeFile'
]
// fs.mkdtemp() was added in Node.js v5.10.0, so check if it exists
typeof fs.mkdtemp === 'function' && api.push('mkdtemp')

// Export all keys:
Object.keys(fs).forEach(key => {
  exports[key] = fs[key]
})

// Universalify async methods:
api.forEach(method => {
  exports[method] = u(fs[method])
})

// We differ from mz/fs in that we still ship the old, broken, fs.exists()
// since we are a drop-in replacement for the native module
exports.exists = function (filename, callback) {
  if (typeof callback === 'function') {
    return fs.exists(filename, callback)
  }
  return new Promise(resolve => {
    return fs.exists(filename, resolve)
  })
}

// fs.read() & fs.write need special treatment due to multiple callback args

exports.read = function (fd, buffer, offset, length, position, callback) {
  if (typeof callback === 'function') {
    return fs.read(fd, buffer, offset, length, position, callback)
  }
  return new Promise((resolve, reject) => {
    fs.read(fd, buffer, offset, length, position, (err, bytesRead, buffer) => {
      if (err) return reject(err)
      resolve({ bytesRead, buffer })
    })
  })
}

// Function signature can be
// fs.write(fd, buffer[, offset[, length[, position]]], callback)
// OR
// fs.write(fd, string[, position[, encoding]], callback)
// so we need to handle both cases
exports.write = function (fd, buffer, a, b, c, callback) {
  if (typeof arguments[arguments.length - 1] === 'function') {
    return fs.write(fd, buffer, a, b, c, callback)
  }

  // Check for old, depricated fs.write(fd, string[, position[, encoding]], callback)
  if (typeof buffer === 'string') {
    return new Promise((resolve, reject) => {
      fs.write(fd, buffer, a, b, (err, bytesWritten, buffer) => {
        if (err) return reject(err)
        resolve({ bytesWritten, buffer })
      })
    })
  }

  return new Promise((resolve, reject) => {
    fs.write(fd, buffer, a, b, c, (err, bytesWritten, buffer) => {
      if (err) return reject(err)
      resolve({ bytesWritten, buffer })
    })
  })
}