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JavaScript implementation of the TrueSkill algorithm
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TrueSkill is a skill-based ranking system developed by Microsoft Research. It is used by Xbox LIVE to rank and match players. This package implements TrueSkill in JavaScript as an npm package.


Import the module.

var trueskill = require('trueskill');


TrueSkill encodes parameters about the game.

var env = new trueskill.TrueSkill({
  mu: 25,
  sigma: 25 / 3,
  beta: 25 / 6,
  tau: 25 / 300,
  draw: 0.1


  • mu: the initial mean
  • sigma: the initial standard deviation
  • beta: the number of skill points to guarantee an 80% chance of winning
  • tau: the dynamics factor
  • draw: the probability of a draw in [0, 1]


Rating encodes a player's rating.

var rating = new trueskill.Rating(mu, sigma);


  • mu: the mean value of the rating
  • sigma: the standard deviation of the rating


transformRatings calculates updated ratings based prior ratings and the outcome of a game.

var newRatings = env.transformRatings(
    { 'p1': r1, 'p2': r2 },
    { 'p3': r3, 'p4': r4 }
  [ 2, 1 ]);


  • teams: an array of teams, where each team is a map of player id to prior rating
  • ranks: the ranks of the teams, where 1 indicates the winner


matchQuality calculates the quality of a match as a function of the probability of all teams drawing.

var quality = env.matchQuality(
    { 'p1': r1, 'p2': r2 },
    { 'p3': r3, 'p4': r4 }

where the teams parameter is the same as transformRatings.


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