@matsos/expected-wins/src/index.js

A simple javascript library based on a formula developed by baseball statistician Bill James. It attempts to determine the number of games a team should have won, by comparing the total number of runs a team has scored to the number of runs it has allowed

module.exports = function basicExpectedWins(runsScored, runsAllowed) {
  // make sure both inputs are numbers
  if (typeof runsScored !== "number" || typeof runsAllowed !== "number")
    throw new TypeError("Both runsScored and runsAllowed should be numbers");

  // calculate the expected win percentage
  result = 1 / (1 + Math.pow(runsAllowed / runsScored, 2));

  // round the pecentage to 3 decimal places and return it
  return Math.round(result * 1000) / 1000;
};