elo

Calculates Elo rating.

Node Repository

https://www.npmjs.com/package/@pelevesque/elo

Installation

npm install @pelevesque/elo

Tests

Command	Description
npm test or npm run test	All Tests Below
npm run cover	Standard Style
npm run standard	Coverage
npm run unit	Unit Tests

Methods

• getExpectedScore
• getRatingDelta
• getRating
• getOutcome

Usage

Initialization

When initializing elo, you can set the k and deviation factors. If not set, the defaults will be k = 24 and d = 400.

format: new Elo(k, d)

// initialize without parameters
const elo = new Elo()

// initialize with parameters
const elo = new Elo(32, 200)

Modifying parameters after initialization

You can modify the k and deviation factors after initialization by setting them directly.

elo.k = 32
elo.d = 800

Get Expected Score

getExpectedScore returns the expected score of a match up.

You can optionally override the deviation factor set in the constructor.

format: getExpectedScore(ratingA, ratingB, deviation)

// without overriding the deviation factor
const expectedScore = elo.getExpectedScore(2000, 1500)

// overriding the deviation factor
const expectedScore = elo.getExpectedScore(2000, 1500, 800)

Get Rating Delta

getRatingDelta returns the change in rating.

You can optionally override the k and deviation factors set in the constructor.

The score is between 0 (loss) and 1 (win). 0.5 is a tie.

format: getRatingDelta(ratingA, ratingB, score, k, deviation)

// without overriding the k and deviation factors
const ratingDelta = elo.getRatingDelta(2000, 1600, 1)

// overriding the k and deviation factors
const ratingDelta = elo.getRatingDelta(2000, 1500, 0.5, 32, 800)

Get Rating

getRatingDelta returns the new rating for player A.

You can optionally override the k and deviation factors set in the constructor.

The score is between 0 (loss) and 1 (win). 0.5 is a tie.

format: getRating(ratingA, ratingB, score, k, deviation)

// without overriding the k and deviation factors
const rating = elo.getRating(2000, 1600, 1)

// overriding the k and deviation factors
const rating = elo.getRating(2000, 1500, 0.5, 32, 800)

Get Outcome

getOutcome is elo's most useful method. It returns an object with the rating delta and new rating for both players.

You can optionally override the k and deviation factors set in the constructor.

The score is between 0 (loss) and 1 (win). 0.5 is a tie.

format: getOutcome(ratingA, ratingB, score, k, deviation)

// without overriding the k and deviation factors
const outcomeObject = elo.getOutcome(2000, 1600, 1)

// overriding the k and deviation factors
const outcomeObject = elo.getOutcome(2000, 1500, 0.5, 32, 800)

// outcome structure example
{
  a: {
    delta: 12,
    rating: 2012
  },
  b: {
    delta: -12,
    rating: 1988
  }
}

Example

const elo = new Elo(12)
const outcome = elo.getOutcome(2000, 2050, 1)
const newRatingA = outcome.a.rating