EloCalculator.js

function eloRating(player, opponent, score, K = 20) {
  // Score is 1 if player won, 0 if opponent won, and 0.5 if a draw
  // Information: https://www.omnicalculator.com/sports/elo
  let difference = K * (score - (1 / (1 + 10**((opponent - player)/400))))
  difference = Math.round(Math.abs(difference))

  if (score >= 1) {
      player += difference
      opponent -= difference
  } else if (score <= 0) {
      player -= difference
      opponent += difference
  } else {
      if (player < opponent) {
          player += difference
          opponent -= difference
      } else {
          player -= difference
          opponent += difference
      }
  }

  return { player: player, opponent: opponent, difference: difference}
}