functional fold in javascript

foldl.js

var foldl = function(f, init, l) {
  switch(l.length) {
  case 0: return init;
  default: return arguments.callee(
    f, f(init, l[0]), l.slice(1)
  );
  }
};

foldr.js

var foldr = function(f, init, l) {
  return foldl(f, init, l.reverse());
};

(function() {
  var foldl = function(f, ac, xs) {
    return (xs.length === 0) && ac
      || foldl(f, f(ac, xs[0]), xs.slice(1));
  };
  console.log(
    foldl((x,y) => x+y, 0, [1,2,3]) === 6 // true
  );
})();