defer.js

var defer = function () {
  var value,
      resolution,
      handlers = {};

  var fulfill = function () {
    if (resolution) {
      var handler = handlers[resolution];
      if (handler) {
        handler();
      }
    }
  };

  var wrap = function (c, d) {
    return function () {
      c = c || function () {};

      if (resolution === 'rejected') {
        c(value);
        return d.reject(value);
      }

      var next;
      try {
        next = c(value);
      }
      catch (e) {
        return d.reject(e);
      }

      if (next) {
        if (next.then) {
          return next.then(function (resp) {
            return d.resolve(resp);
          }, function (err) {
            return d.reject(err);
          });
        }
      }

      return d.resolve(next);
    };
  };

  var resolve = function (v) {
    value = v;
    resolution = 'resolved';
    fulfill();
    return this;
  };

  var reject = function (v) {
    value = v;
    resolution = 'rejected';
    fulfill();
    return this;
  };

  var then = function (done, fail) {
    var deferred = defer();
    handlers.resolved = wrap(done, deferred);
    handlers.rejected = wrap(fail, deferred);

    fulfill();

    return deferred.promise;
  };

  var promise = {
    then: then
  };

  return {
    promise: promise,
    then: then,
    resolve: resolve,
    reject: reject
  };
};

nice :)