Promise.js

function Promise(executor) {
  var successCallbacks = [];
  var failureCallbacks = [];
  var state = 'pending';

  var resolve = function(resolutionValue) {
    if (state != 'pending') { return; }
    successCallbacks.forEach(f => f(resolutionValue));
    state = 'resolved';
    result = resolutionValue;
  }

  var reject = function(rejectionValue) {
    if (state != 'pending') { return; }
    failureCallbacks.forEach(f => f(rejectionValue));
    state = 'rejected';
    result = rejectionValue;
  }

  this.then = function(s, f) {
    s = s || (x => x);
    f = f || (x => x);
    return new Promise(function(resolve, reject) {
      var successCallback = function(resolution) {
        var callbackResult = s(resolution);
        if (typeof callbackResult == 'object' && 'then' in callbackResult) {
          callbackResult.then(resolve, reject);
        } else {
          resolve(callbackResult);
        }
      };

      var failureCallback = function(rejection) {
        var callbackResult = f(rejection);
        if (typeof callbackResult == 'object' && 'then' in callbackResult) {
          callbackResult.then(resolve, reject);
        } else {
          reject(callbackResult);
        }
      };

      if (state != 'pending') {
        state == 'resolved' ? successCallback(result) : failureCallback(result);
      } else {
        successCallbacks.push(successCallback);
        failureCallbacks.push(failureCallback);
      }
    });
  };

  executor(resolve, reject);
}

export default Promise;