futures.js

var Future = require('fluture')
var fs = require('fs')

var readFile = filename => {
  return new Future((reject, resolve) =>
    fs.readFile(filename, 'utf-8', (err, data) =>
      err ? reject(err) : resolve(data)
    )
  )
}

var writeFile = (filename, content) => {
  return new Future((reject, resolve) =>
    fs.writeFile(filename, content, 'utf-8', err =>
      err ? reject(err) : resolve(content)
    )
  )
}

readFile('./version.txt')
  .map(x => parseInt(x, 10))
  .map(x => x + 1)
  .chain(newVersion => writeFile('./version.txt', newVersion))
  .fork(
    err => console.error('could not save - ' + err),
    number => console.log('saved new version - ' + number)
  )