family-probability.js

function randInt (n) {
    return Math.floor(Math.random() * n)
}

function randChoice (array) {
    return array[randInt(array.length)]
}

function generateFamily (numberOfKids, possibleGenders) {
    const result = []
    for (let i = 0; i < numberOfKids; i++) {
        result.push(randChoice(possibleGenders))
    }
    return result
}

function generatePopulation (numberOfFamilies, numberOfKidsPerFamily, possibleGenders) {
    const result = []
    for (let i = 0; i < numberOfFamilies; i++) {
        result.push(generateFamily(numberOfKidsPerFamily, possibleGenders))
    }
    return result
}

const KIDS_PER_FAMILY = 2
const FAMILIES_TOTAL = 10000
const GENDERS = [ 'male', 'female' ] // we all know there are really only two ;)
const population = generatePopulation(FAMILIES_TOTAL, KIDS_PER_FAMILY, GENDERS)

function familyHasSpecificGenderChild (family, gender) {
    return family.includes(gender)
}

function allFamilyChildrenAreSameGender (family, gender) {
    return family.filter(c => c === gender).length === family.length
}

const genderToTry = 'male'
const stats = {
    totalFamilesTried: 0,
    totalFamilesWithMalesTried: 0,
    femaleOnlyFamiliesFound: 0,
    maleOnlyFamiliesFound: 0,
    mixedFamiliesFound: 0,
}

function theTry (population, gender) {
    const randomFamily = randChoice(population)
    stats.totalFamilesTried++
    if (!familyHasSpecificGenderChild(randomFamily, gender)) {
        stats.femaleOnlyFamiliesFound++
        return
    } else {
        stats.totalFamilesWithMalesTried++
        if (allFamilyChildrenAreSameGender(randomFamily, gender)) {
            stats.maleOnlyFamiliesFound++
        } else {
            stats.mixedFamiliesFound++
        }
    }
}

const numberOfTries = 5000

for (let i = 0; i < numberOfTries; i++) {
    theTry(population, genderToTry)
}

console.log(stats)

console.log('if there is a male child, all are males with probability...', stats.maleOnlyFamiliesFound / stats.totalFamilesWithMalesTried)