Monty Hall Problem Simulation

montyhall.js

const winsIfSwitched = [];
const winsIfStayed = [];

function monty () {
  const doorWithCar = Math.floor(Math.random() * 3);
  const guessedDoor = Math.floor(Math.random() * 3);

  // Host removes a door that...
  const removedDoor = [0,0,0].reduce((result, n, index) => {
    // 1) contestant didn't pick and...
    if (index === guessedDoor) return result;
    // 2) doesn't have a car
    if (index === doorWithCar) return result;

    return index;
  }, -1);

  // Contestant has a choice to switch tho a door that...
  const remainingDoor = [0,0,0].reduce((result, n, index) => {
    // 1) contestant didn't already pick and...
    if (index === guessedDoor) return result;
    // 2) hasn't been removed by host
    if (index === removedDoor) return result;

    return index;
  }, -1);

  // Did contestant win if they stayed with the original guess?
  const stayedResult = guessedDoor === doorWithCar;

  // Did contestant win if they switched to the remaining door?
  const switchedResult = remainingDoor === doorWithCar;

  winsIfStayed.push(stayedResult);
  winsIfSwitched.push(switchedResult);
}

const numTrials = 10000000;
console.log(`Running ${numTrials} trials...`);
for (i = 0; i < numTrials; i++) monty();

console.log('Win rate if stayed', winsIfStayed.filter(Boolean).length / winsIfStayed.length);
console.log('Win rate if switched', winsIfSwitched.filter(Boolean).length / winsIfSwitched.length);