starx

Motivation

starx allows you to use ECMAScript 6 generators to perform flow control. (The 'star' in the name is inspired from the declaration of a generator function, i.e. function*() {...} while 'x' stands for 'executor'.)

Basically, no more callback hells with cumbersome exception handling...

request(url1, function(err, res1) {
  if (err) return print(err)
  request(url2, function(err, res2) {
    if (err) return print(err)
    request(url3, function(err, res3) {
      if (err) return print(err)
      print(size([res1, res2, res3]) + " bytes")
    })
  })
})

With starx, you can write:

starx(function*() {
  try {
    var res1 = yield request(url1)
    var res2 = yield request(url2)
    var res3 = yield request(url3)
    print(size([res1, res2, res3]) + " bytes")
  } catch (err) {
    print(err)
  }
})()

How it works

Invoking starx() on a generator returns an executor. You can create multiple executors for the same generator instance.

This executor, when invoked, keeps calling generator.next() until the generator is exhausted. Each call to generator.next() returns the next value yielded (or returned) from inside the generator. For starx to work, this value must be "yieldable", i.e. one of the followings:

• A functions whose only argument is a callback accepting (err, val) (see yieldable functions)
• A promise (anything with a then(callback, errback))
• A value (primitive, object, null)
• Another executor created by starx
• A generator or iterator
• An array of the aboves (nesting okay)

Yieldable functions

If a function takes more than one argument, you can use starx.yieldable(fn) to convert it to a compliant form. For example:

If fn is function(arg1, arg2, cb) {...}, invoking starx.yieldable(fn) returns a new function with the following form:

var newFn = function(arg1, arg2) { 
    fn(arg1, arg2, fakeCb)
    return function(cb) {...}
}

Invoking newFn(arg1, arg2) returns a function accepting a callback, which as mentioned, is compliant with starx and thus can be yielded, like so:

starx(function*() {
    yield newFn(arg1, arg2)
})()

Most NodeJS functions (built-in and libraries) can be converted to a yieldable function using this approach.

Examples

Read file

starx = require('starx')
readFile = starx.yieldable(require('fs').readFile)
 
var generator = function*() {
  var content = yield readFile(__filename, 'utf8')
  console.log(content)
}
var executor = starx(generator)
executor()

Or simply:

starx(function*() {
  var content = yield readFile(__filename, 'utf8')
  console.log(content)
})()

Serial download

starx = require('starx')
request = starx.yieldable(require('request'))
 
starx(function*() {
  var res1 = yield request("https://www.google.com/")
  var res2 = yield request("https://www.bing.com/")
  var res3 = yield request("https://www.yahoo.com/")
  console.log(size([res1, res2, res3]), "bytes")
})()
 
function size(responses) {
  return responses.reduce(function(a, c) {
    return a + c.body.length
  }, 0)
}

Parallel download

starx(function*() {
  var r1 = request("https://www.google.com/")
  var r2 = request("https://www.bing.com/")
  var r3 = request("https://www.yahoo.com/")
  var res = yield [r1, r2, r3]
  console.log(size(res), "bytes")
})()

A DRYer version

starx(function*() {
  var res = yield [urls].map(function(url) {
    return request(url)
  })
  console.log(size(res), "bytes")
})()

You might think we could have written:

yield [urls].map(request)

But that wouldn't work. The reason is map invoke request with not just the element, but also its index and the original array. Because yieldable passes through all arguments by default, request would end up being invoked with those 3 arguments while it actually expects the second argument to be a callback.

To make this work, either explicitly invoke request with url as the previous example or provide a second argument to yieldable: argCount. If argCount is true, yieldable limits the number of arguments passed through to be fn.length-1. If argCount is a number, yieldable limits the number of arguments to be argCount-1.

We can revise the previous example as follows:

request = starx.yieldable(require('request'), true /* argCount */)
starx(function*() {
  var res = yield [urls].map(request)
  console.log(size(res), "bytes")
})()

Install

NPM

npm install starx

Bower

bower install starx

• Node >= 0.11.6 (run with --harmony or --harmony-generators)
• Chrome >= 28 (turn on experimental flag)
• Firefox >= 27
• Or use Google Traceur