flip.js

let flip = (flopped => {
  return function flip(fn) {
    if (flopped.has(fn)) {
      return flopped.get(fn);
    }
    
    flopped.set(fn, flipped);
    flopped.set(flipped, fn);

    return flipped;

    function flipped(...rest) {
      return fn.apply(this, rest.reverse());
    }
  }
})(new WeakMap());

function partition(list, predicate) {
  return list.reduce((memo, subject) => {
    memo[1 - predicate(subject)].push(subject);
    return memo;
  }, [[],[]]);
}

let [ more, less ] = flip(partition)(it => it > 5, [0,1,2,3,4]);

assert(partition === flip(flip(partition)));
assert(flip(partition) === flip(flip(flip(partition))));

function assert(condition) { if (!condition) throw Error("assertion failed") }
undefined