Angular, reuse a $q promise if not resolved.

example.js

var factoryItem = ["$q", function ($q) {

  var factory = {};

  var pendingPromises = {};

  factory.test = function (identifier) {

    // Assigns d to the variable that will contain the promise if pending.
    var d = pendingPromises[identifier];

    // Check to see if d is not null or undefined, and checks to 
    // see if the promise's state's status is 0 (not resolved/rejected)
    if (d && d.promise.$$state.status === 0) {
      console.log("Promise already pending, return it.");
      return d.promise;
    }

    console.log("Pending promise not found, create a new one.");
    d = pendingPromises[identifier] = $q.defer();

    // Create a 5 second delay before resolving the promise simulating an AJAX call
    setTimeout(function () {
      d.resolve("Promise has been resolved.");
    }, 5000);

    // Return the new promise.
    return d.promise.finally(function () {
      // clean up the collection object since the promise was resolved/rejected.
      console.log("Promise resolved/rejected, purge the promise from the collection.");
      delete pendingPromises[identifier];
    });
  };

  return factory;
}];