gistfile1.gs

function controlStuff(){
  var listOfThings = loadThings()
  for(thing in listOfThings.whereTypeIs(CanDoSomething)){
    thing.doSomething()
  }
}

// or just...

function controlStuff(){
  var listOfThings = loadThings()
  listOfThings.whereTypeIs(CanDoSomething)*.doSomething()
}

interface CanDoSomething {
  function doSomething()
}