Element Poller + chooser - Streams based, returns all results (found + not found) http://jsbin.com/calekubavi/edit?js,console,output

domRace.js

// requires RX.js

const NUMBER_OF_PLACEMENTS_REQUIRED = 1;
const MS_TO_WAIT = 7000;
const selectors = ['.wibble', '.bleuch'];

function checkElement(selector, observer) {
  var elementFound = document.querySelector(selector) !== null;
  console.log(`el ${selector} = found : ${elementFound}`);
  
  observer.next({found: elementFound, selector: selector});
  
  if (elementFound) {
    observer.completed();
  }
}

function createElementPoller(selector) {
  return Rx.Observable.create(function (observer) {
    const intervalId = setInterval(() => checkElement(selector, observer), 1000);
  
    return function unsubscribe() {
      if (intervalId) {
        clearInterval(intervalId);      
      }
    };
  });
}

const pollers = selectors.map(createElementPoller);

function areElementsFound(placementSources) {
  console.log('** element changes pushed');
  const foundPlacements = placementSources.filter(result => result.found);
  return foundPlacements.length >= NUMBER_OF_PLACEMENTS_REQUIRED;
}

var allElementsSource = Rx.Observable.combineLatest(pollers)
                                     .distinct()
                                     .filter(areElementsFound)
                                     .take(1)
                                     .timeout(MS_TO_WAIT, 'give up');

var elementListener = allElementsSource.subscribe(
  function onNext(outputArray){console.log('output is ', outputArray);},
  function onError(err){console.error(err);},
  function onComplete(outputArray){console.log('done :)');}
);

// ========================
// next part is for testing it
// you'd normally just let the DOM do this
// ========================
(function addTestElements() {
  function addElement(name, delay) {
    setTimeout(function() {
      var p = document.createElement('p');
      p.innerText = name;
      p.className = name;
      console.log(`^^ adding .${name}`)
      document.body.appendChild(p);
    }, delay);
  };
  addElement('wibble', 2000);
  addElement('bleuch', 4000); 
}());