Manual implementation of Promise

promise.js

function Promise(fn) {
  var callback = null;
  this.then = function(cb) {
    callback = cb;
  };

  function resolve(value) {
    // force callback to be called in the next
    // iteration of the event loop, giving
    // callback a chance to be set by then()
    setTimeout(function() {
      callback(value);
    }, 1);
  }

  fn(resolve);
}