The Elo rating system is a method for calculating the relative skill levels of players in zero-sum games such as chess. It is named after its creator Arpad Elo, a Hungarian-American physics professor - wikipedia

npm install elo-rating-system

Usage

const { Elo } = require('elo-rating-system');

const elo = new Elo({ k: 20, rating: 2000 }); // override the defaults (k: 20, rating: 1200)

.change() - rating change based on opponentRating and result

const { change, newRating } = elo.change(2200, 1); // { change: 15.19..., newRating: 2015 }

.probability() - decimal probability of winning vs opponentRating

const probability = elo.probability(2200); // 0.24 (or 24%)

.performance() - performance rating from an array of game results

const perf = elo.performance([
    { opponentRating: 2200, result: 1 },
    { opponentRating: 2100, result: 0.5 },
    { opponentRating: 2600, result: 0 },
]);
// { games: 3, change: [...], ratings: [...], tpr: 2233.33 }

Results use 1 for a win, 0.5 for a draw, and 0 for a loss.

Dynamic K-Factor

Pass a function as k to vary the K-factor based on the player's rating:

const elo = new Elo({
    rating: 2000,
    k: (rating) => {
        if (rating < 2400) return 40;
        return 10;
    },
});

Tests

npm test