js-proxy-yatzy.js

function Yatzy(dice) {
  const callDirectory = {
    ones: 1,
    twos: 2,
    threes: 3,
    fours: 4,
    fives: 5,
    sixes: 6
  }
  const sum = function(dice, num) { 
    return dice.reduce(function(acc, el) {
      return (el == num) ? acc += el : acc
    }, 0) 
  }
  const sumHandler = {
    get: function(target, name) {
      return sum(dice, target[name])
    }
  }
  const sumProxy = new Proxy(callDirectory, sumHandler);
  return sumProxy;
}

Invocations:

Yatzy([1,1,1,2,5]).ones
=> 3
Yatzy([3,5,4,3,2]).threes
=> 6
Yatzy([6,6,6,6,2]).sixes
=> 24