// Event composition util, example usage:
//
//     var a = eventComposeHelper(2, () => console.log('DONE'))
//     a[0]()
//     a[1]()  // DONE logged
//
function eventComposeHelper(numCallbacksNeeded, whenDone) {
  const callbacksCalled = Array(numCallbacksNeeded).fill(false);

  function buildCallbackHandler(index) {
    return function() {
      callbacksCalled[index] = true;

      if (callbacksCalled.every(x => x === true)) {
        whenDone();
        callbacksCalled.fill(false);  // if we need to run `whenDone` multiple times
      }
    }
  }

  return Array(numCallbacksNeeded).fill(null).map((x, i) => buildCallbackHandler(i));
}