gistfile1.js

var exists = function(x) { return x != null; };

var truthy = function(x) { 
  return (x !== false) && exists(x); 
};

var _filter = function(fun) {
  return function(coll) {
    var results = [];
    if (coll == null) return results;
    coll.forEach(function(value, index, list) {
      if (truthy(fun.call(null, value))) results.push(value);
    });
    return results;
  };
};
 
function filter(fun) {
  var coll = arguments[1];
  var f1 = _filter(fun);
 
  if (exists(coll)) return f1(coll);
  else return f1;
}

Yep. I see how this would be a bit better.