birthday problem calculator

gistfile1.js

let birthday = n => {let r=1;for(let i=1;i<n;i++){r*=(366-i)/366}return 1-r}

Explanation:

// gives the chance of two people having the same birthday, given the number of people
function birthday(numPeople) {
  // the probability of have no match for the first person is 1, since there is no one to match to
  let failChance = 1;
  for (let checkedPpl = 1; checkedPpl < numPeople; checkedPpl++) {
    const remainingBirthdays = 366 - checkedPpl
    const chanceOfNewBirthday = remainingBirthdays / 366
    failChance *= chanceOfNewBirthday
  }
  return 1 - failChance
}