Combine variances of subsets

combineVariances.js

/**
 * statistics of a set
 * @typedef {object} stat
 * @prop {numher} count
 * @prop {number} average
 * @prop {number} variance
 */

/**
 * combine variances
 * @param {stat[]} stats
 * @returns {stat}
 */
function combineVariances(stats){
    let sum = 0;
    let count = 0;
    let sumVariances = 0;

    stats.forEach(stat => {
        count += stat.count;
        sum += stat.average * stat.count;
    });
    
    const average = sum / count;
    
    stats.forEach(stat => {
        sumVariances += (Math.pow(stat.average - average, 2) + stat.variance)  * stat.count;
    });

    return {
        count: count,
        average: average,
        variance: sumVariances / count
    }
}