example-filter.js

class Cat {
  speak() { return "meow"; }
}

function process(cat) { 
  return cat.speak(); 
}

class Ball {
}

function processAnything(x) {
  if(x instanceof Cat) {
    return process(x);
  }
}

processAnything(new Cat()); // => "meow"
processAnything(new Ball()); // => undefined