cancelable promise prototype

gistfile1.txt

function cancellablePromise(executor) {
  var deferred = Promise.defer();
  var cancelled = false;
  var promise = new Promise(executor).then(function(result) {
    if (!cancelled) deferred.resolve();
    else return arguments[0]
  });

  var actualPromise = Promise.all(deferred.promise, promise)
    .then(function(result) {
      return result.pop(); // return the actual result
    })
  
  return {
    cancel: function(reason) {
      cancelled = true;
      if (reason) {
        deferred.reject(new Error(reason))
      }
    },
    promise: actualPromise,
    then: actualPromise.then.bind(actualPromise)
  }
}