Promises Caching

promises_caching.js

var inProgressUrls = {};

function grabScreenshot(url) {
  var inProgress = inProgressUrls[url];
  if (inProgress) {
    return inProgress;
  }

  var promise = new Promise(function(fulfill, reject) {
    ... grab screenshot ...

    // Once done, remove the Promise from the map.
    delete inProgressUrls[url];
  });

  inProgressUrls[url] = promise;

  return promise;
}