Example of how promises cleaned up my node.js code

app-callbacks.js

app.post('/books/:bookId/files', function(req, res) {
  File.addFilesToBook(req.params.bookId, req.files.file, function(err, files) {
    if (err instanceof Error) return res.send(500, err.message)
    res.json(files)
  })
})

app-promises.js

app.post('/books/:bookId/files', function(req, res) {
  File.addFileToBook(req.params.bookId, req.files.file)
  .then(send(res), err(res))
})

File-callbacks.js

export function addFileToBook(bookId:string, uploadedFile:IUploadFile, cb:(err:Error, file:IFile) => void) {
  var file = toFile(bookId, uploadedFile)
  uploadToUrl(file, uploadedFile, function(err) {
    if (err) return cb(err, null)
    insert(file).run(function(err) {
      if (err instanceof Error) return cb(err, null)
      cb(null, file)
    })
  })
}

File-promises.js

export function addFileToBook(bookId:string, uploadedFile:IUploadFile):q.IPromise {
  var file = toFile(bookId, uploadedFile)
  return uploadToUrl(file, uploadedFile)
  .then(() => db.run(insert(file))) // return a promise here, will happen async
  .then(() => file) // makes it finally "return" the file
}