Why type-checking (even in an environment like Phantom) is virtually impossible in Javascript

typecheck.js

var foo = {
  filter: function () {}
};

var bar = [];

// return filtered array or object
function filterer(baz) {
  return baz.filter();
}

// call filterer with an object that has a `filter` function
filterer(foo);

// on some user interaction that wouldn't be executed by phantom,
// call `filterer` with a completely different type of argument
// that may or may not have a `filter` function
document.querySelector('#some-button').addEventListener('click', function () {
  filterer(bar);
});