Creating Promises

promise.js

// This is `promiseSetTimeout` which is a wrapper around `setTimeout`
// `promiseSetTimeout` manually builds out the `then` function to its result
// in order to demonstrate how promises work
var promiseSetTimeout = function(time) {
  var result = {};
  var callbacks = [];

  // `then` is a method that will take the callback and store
  // it to be used later. In a promise-less world, this would
  // be the callback function inserted in the call to `setTimeout`.
  result.then = function(callback) {
    callbacks.push(callback);
  }

  setTimeout(function() {
    callbacks.forEach(function(cb) {
      cb();
    })
  }, time);

  return result;
}

// `promisify` takes a function that runs asynchronously via callbacks and
// returns a function that returns a promise
var promisify = function(func) {

  // Create a function that will return a promise
  // The input function takes two arguments with the 2nd one being
  // a callback
  var promiseFunc = function(arg1) {
    // result will have 2 properties. Value (result of the async call), and
    // `then` which is the promise-based function
    var result = {};
    var callbacks = [];

    func(arg1, function(value) {
    // Execute all of the callbacks that were added to the callbacks array
      result.value = value;
      callbacks.forEach(function(cb) {
        cb(value);
      });
    });

    // Creation of the `then` function which will add a callback to
    // our list
    result.then = function(callback) {
      callbacks.push(callback);
    };
    return result;
  };

  return promiseFunc;
}