promise.js

// Credits: https://github.com/city41
function Promise(fn) {
  var state = 'pending';
  var value;
  var deferred = null;

  function resolve(newValue) {
      try {
          if (newValue && typeof newValue.then === 'function') {
              newValue.then(resolve, reject);
              return;
          }
          state = 'resolved';
          value = newValue;

          if (deferred) {
              handle(deferred);
          }
      } catch (e) {
          reject(e);
      }
  }

  function reject(reason) {
      state = 'rejected';
      value = reason;

      if (deferred) {
          handle(deferred);
      }
  }

  function handle(handler) {
      if (state === 'pending') {
          deferred = handler;
          return;
      }
      setTimeout(function(){
        var handlerCallback;

        if (state === 'resolved') {
            handlerCallback = handler.onResolved;
        } else {
            handlerCallback = handler.onRejected;
        }

        if (!handlerCallback) {
            if (state === 'resolved') {
                handler.resolve(value);
            } else {
                handler.reject(value);
            }

            return;
        }

        var ret;
        try {
            ret = handlerCallback(value);
            handler.resolve(ret);
        } catch (e) {
            handler.reject(e);
        }
      }, 1)
  }

  this.then = function (onResolved, onRejected) {
      return new Promise(function (resolve, reject) {
          handle({
              onResolved: onResolved,
              onRejected: onRejected,
              resolve: resolve,
              reject: reject
          });
      });
  };

  fn(resolve, reject);
}