Convert any NodeJS async function into a Promise

denodify.js

module.exports = function denodify(fn) {
  return function() {
    var self = this;
    var args = Array.prototype.slice.call(arguments);

    return new Promise(function(resolve, reject) {
      args.push(function(error, result) {
        error ? reject(error) : resolve(result);
      });

      fn.apply(self, args);
    });
  }
}

example-co.js

var fs = require("fs");
var denodify = require("denodify");
var co = require("co");

var readFile = denodify(fs.readFile);

co(function *() {
  var hosts = yield readFile("/etc/hosts");
  var passwd = yield readFile("/etc/passwd");
  console.log("hosts:", hosts.toString());
  console.log("passwd: ", passwd.toString());
});

example.js

var fs = require("fs");
var denodify = require("denodify");

var readFile = denodify(fs.readFile);

readFile("./denodifys.js")
  .then(function(file) { return file.toString() })
  .then(function(file) { 
    return file.length > 5 ? Promise.reject("Arquivo muito pequeno") : file.length; 
  })
  .then(console.log, console.error)

Promise.all([readFile("/etc/hosts"), readFile("/etc/passwd")]).then(function(files) {
  var hosts = console.log("hosts:", files[0].toString());
  var passwd = console.log("passwd:", files[1].toString());
});