elo.js

const elo = ([...ratings], kFactor = 32, selfRating) => {
  const [a, b] = ratings;
  const expectedScore = (self, opponent) =>
    1 / (1 + 10 ** ((opponent - self) / 400));
  const newRating = (rating, i) =>
    (selfRating || rating) +
    kFactor * (i - expectedScore(i ? a : b, i ? b : a));
  if (ratings.length === 2)
    return [newRating(a, 1), newRating(b, 0)];

  for (let i = 0, len = ratings.length; i < len; i++) {
    let j = i;
    while (j < len - 1) {
      j++;
      [ratings[i], ratings[j]] = elo(
        [ratings[i], ratings[j]],
        kFactor,
      );
    }
  }
  return ratings;
};
// Standard 1v1s
elo([1200, 1200]); // [1216, 1184]
elo([1200, 1200], 64); // [1232, 1168]
// 4 player FFA, all same rank
elo([1200, 1200, 1200, 1200]).map(Math.round); // [1246, 1215, 1185, 1154]
/*
For teams, each rating can adjusted based on own team's average rating vs.
average rating of opposing team, with the score being added to their
own individual rating by supplying it as the third argument.
*/