1.0.0 • Public • Published

yield-callback - run async/callback functions in a sync style, with generators

This library makes it easy to write code that makes serial calls to async functions that use callbacks in the typical "Node.js" style - eg, cb(err, data). Forget the Callback Hell - and write code that looks like it's synchronous, but isn't. Thanks to the magic that is JavaScript Generators.


Say you want to write a function readFile() which reads the contents of a file using the fs module. Here's how you would do it using Node.js's low-level fs module functions, in a fairly typical pyramid-of-callback-hell-doom:

function readFile (fileName, cb) {
  fs.open(fileName, 'r', function (err, fd) {
    if (err) return cb(err)
    fs.fstat(fd, function (err, stats) {
      if (err) return cb(err)
      const buffer = new Buffer(stats.size)
      fs.read(fd, buffer, 0, buffer.length, 0, function (err, bytesRead, bufferRead) {
        if (err) return cb(err)
        if (bytesRead !== buffer.length) return cb(new Error('EMOREFILE'))
        fs.close(fd, function (err) {
          if (err) return cb(err)
          cb(null, bufferRead)

With yield-callback, you can write it like this instead:

function * readFileGen (fileName, cb) {
  const fd = yield fs.open(fileName, 'r', cb)
  if (cb.err) return cb.err
  const stats = yield fs.fstat(fd, cb)
  if (cb.err) return cb.err
  const buffer = new Buffer(stats.size)
  const bytesReadBuffer = yield fs.read(fd, buffer, 0, buffer.length, 0, cb)
  if (cb.err) return cb.err
  const bytesRead = bytesReadBuffer[0]
  const bufferRead = bytesReadBuffer[1]
  if (bytesRead !== buffer.length) return new Error('EMOREFILE')
  yield fs.close(fd, cb)
  if (cb.err) return cb.err
  return bufferRead

Your readFile() function will actually be implemented as a generator function. Note the generator function takes a cb parameter at the end, but it it works differently than a typical Node.js callback. You pass that cb parameter as the callback for async functions called inside the generator function, used within yield expressions, and cb arranges to yield the callback 'result' argument(s) as the result, and sets the 'error' argument to cb.err:

You can then invoke the generator as below. Note that you pass a "normal" callback function in as the final parameter to run(), which will be invoked when the generator finally returns. If the generator returns an error, the first argument will be that error. Otherwise, the second argument will be set to the return value. Just like a typical Node.js async callback:

yieldCallback.run(readFileGen, fileName, function (err, buffer) {
  if (err) console.log(err)
  else console.log(buffer.toString('utf8'))

Or you can wrap the generator to return a "typical errback" function; in this case, with the same signature and behavior as the pyramid-of-doom readFile() above:

const readFile = yieldCallback(readFileGen)
readFile(fileName, function (err, buffer) {
  if (err) console.log(err)
  else console.log(buffer.toString('utf8'))


npm install yield-callback


This module exports a function which takes a generator function as a parameter and returns a new function with the same signature as the generator function. For the remainder of this document, we'll refer to this function as yieldCallback(), as if you had done a:

const yieldCallback = require('yield-callback')

The returned function takes a callback which will be invoked when the generator returns. The generator itself gets passed a cb argument as it's final parameter, which should be passed as the "callback" function on any async calls that you make, which you should use as expressions in front of a yield:

function * myGenerator(a, b, cb) {
  console.log('waiting(a)', a, 'ms')
  yield setTimeout(cb, a)
  console.log('waiting(b)', b, 'ms')
  yield setTimeout(cb, b)
  return a + b
const myWrapped = yieldCallback(myGenerator)
myWrapped(1000, 2000, function (err, val) {
  console.log('should be 3000:', val)

You can also run a generator directly, via the run function available on the exported function (eg, yieldCallback.run()).

The following are equivalent:

yieldCallback.run(generatorFunction, arg1, arg2, ... callback)

is equivalent to

const wrapped = yieldCallback(generatorFunction)
wrapped(arg1, arg2, ... callback)

API within the generator function

When the generator function is invoked, it's final argument is a special callback function to be used with async callback functions called within the generator. This function can be used as the callback function in an async callback function, if the function is used in a yield expression:

function * genFunction(a, b, cb) {
  yield setTimeout(cb, 1000)
  // code following this comment won't run for 1000 milliseconds

The yield expression returns a value, which is the "result" passed to the callback. The "error" passed to the callback is available as cb.err.

function * genFunction(fileName, cb) {
  // fs.readFile()'s cb: (err, fileContents)
  const fileContents = yield fs.readFile(fileName, 'utf8', cb)
  // the `err` argument of the callback is available in `cb.err`
  if (cb.err) return cb.err
  console.log(fileContents) // print the file contents

If the callback is invoked with a non-null first argument, eg cb(err), the result will be null, and cb.err will be set to that argument.

If the callback is invoked with a single response value, eg cb(null, 1), the result will be the single response value, and cb.err will be null.

If the callback is invoked with multiple response values, eg, cb(null, 1, 2), the result will be an array of the response values, eg [1, 2], and cb.err will be null.

The value that the generator finally returns will be passed to the original callback back passed into the wrapped (or run) function. That callback should have the signature cb(err, data).

If the generator returns an instance of Error, the callback will be invoked with that error as the first parameter.

If the generator return anything else, the callback will be invoked with a null error, and that returned object as the second parameter.

In case you're not sure, or know, that the err object you want to return from a generator isn't an instance of Error, you can use the function cb.errorResult() to wrap your object so that it will be treated as an error result, rather than a non-error object passed as the second callback parameter:

yieldCallback.run(genFunction, aFileName, function outerCB (err, data) {})
function * genFunction(fileName, cb) {
  const data = yield fs.readFile(fileName, 'utf8', cb)
  if (cb.err) return cb.err                  // calls outerCB(err)
  if (cb.err) return cb.errorResult(cb.err)  // also calls outerCB(err),
                                             // even if `cb.err` isn't an
                                             // instance of Error
  return data                                // calls outerCB(null, data)



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npm i yield-callback

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