RSVP.denodeify()

RSVP.denodeify takes a 'node-style' function and returns a function that will return an RSVP.Promise. You can use denodeify in Node.js or the browser when you'd prefer to use promises over using callbacks. For example, denodeify transforms the following:

var fs = require('fs');

fs.readFile('myfile.txt', function(err, data){
  if (err) return handleError(err);
  handleData(data);
});

into:

var fs = require('fs');
var readFile = RSVP.denodeify(fs.readFile);

readFile('myfile.txt').then(handleData, handleError);

If the node function has multiple success parameters, then denodeify just returns the first one:

var request = RSVP.denodeify(require('request'));

request('http://example.com').then(function(res) {
  // ...
});

However, if you need all success parameters, setting denodeify's second parameter to true causes it to return all success parameters as an array:

var request = RSVP.denodeify(require('request'), true);

request('http://example.com').then(function(result) {
  // result[0] -> res
  // result[1] -> body
});

Or if you pass it an array with names it returns the parameters as a hash:

var request = RSVP.denodeify(require('request'), ['res', 'body']);

request('http://example.com').then(function(result) {
  // result.res
  // result.body
});

Sometimes you need to retain the this:

var app = require('express')();
var render = RSVP.denodeify(app.render.bind(app));

The denodified function inherits from the original function. It works in all environments, except IE 10 and below. Consequently all properties of the original function are available to you. However, any properties you change on the denodeified function won't be changed on the original function. Example:

var request = RSVP.denodeify(require('request')),
    cookieJar = request.jar(); // <- Inheritance is used here

request('http://example.com', {jar: cookieJar}).then(function(res) {
  // cookieJar.cookies holds now the cookies returned by example.com
});

Using denodeify makes it easier to compose asynchronous operations instead of using callbacks. For example, instead of:

var fs = require('fs');

fs.readFile('myfile.txt', function(err, data){
  if (err) { ... } // Handle error
  fs.writeFile('myfile2.txt', data, function(err){
    if (err) { ... } // Handle error
    console.log('done')
  });
});

you can chain the operations together using then from the returned promise:

var fs = require('fs');
var readFile = RSVP.denodeify(fs.readFile);
var writeFile = RSVP.denodeify(fs.writeFile);

readFile('myfile.txt').then(function(data){
  return writeFile('myfile2.txt', data);
}).then(function(){
  console.log('done')
}).catch(function(error){
  // Handle error
});