Introduction

Easily mix asynchronous and synchronous code using functional programming.

purpose

Nearly all of our today JavaScript code uses asynchronous operations, whether through Promises, node.js callbacks, or async generators if you are one the the cool kids ™. But that was not always the case! Before the rise of AJAX and node.js, there was almost no asynchronous code in our JavaScript. Thus, nearly all utility functions such as map, filter, or reduce were not designed to handle asynchronous code, and neither were great libraries such as Lodash, Ramda or Underscore. conductor intends to reconcile all these utility functions with asynchronous code, and let you regain control of the execution flow.

example

Let's say we have an array of Star Wars characters, and we want to know who's really from Tatooine, using the awesome Star Wars API:

const characters = [
  { id: 1, name: 'Luke Skywalker', planet_id: 1 },
  { id: 2, name: 'C-3P0', planet_id: 1 },
  { id: 3, name: 'R2-D2', planet_id: 8 },
  { id: 4, name: 'Darth Vader', planet_id: 1 },
  { id: 5, name: 'Leia Organa', planet_id: 2 },
]
const isFromTatooine = character =>
  fetch(`https://swapi.co/planets/${character.planet_id}`)
    .then(response => response.json())
    .then(planet => planet.name === 'Tatooine')

without conductor

const tatooine_residents = await characters.filter(isFromTatooine) // [Luke, C-3PO, R2-D2, Darth Vader, Leia]... wait what???

with conductor

const tatooine_residents = await filter(isFromTatooine, characters) // [Luke, C-3PO, Darth Vader]

The sharp-eyed reader which you are has already noticed that something seems to have gone wrong in our first filtering attempt: the result is a valid array, but last time you checked, R2 and Leia were definitely not from Tatooine! What happens is that Array.prototype.filter has no idea your predicate is asynchronous and does not how to wait for it. But since this predicate still returns a Promise which evaluates to a truthy value, it considers that the values match the predicate and keeps them. Since Array.prototype.filter is itself synchronous, the await keyword is not helping here and only converts the result to a resolved Promise.

In our second attempt, the filter method from conductor automatically detects that our predicate is asynchronous and returns a Promise which will only be resolved when all items are properly filtered. Just add the magical await keyword to wait until the Promise is resolved, and voilà!