gistfile1.js

function promise() {
  var finished = false,
      result = null,
      waiters = [];
  return {
    fulfil: function(res) {
      finished = true;
      result = res;
      forEach(waiters, function(cb) {
        cb(result);
      });
    },
    get: function(callback) {
      if (result) return callback(result);
      waiters.push(callback);
    }
  };
}