Bowling Score Calculator written in JavaScript

bowling-score-calculator.js

const score1 = '165/251/X2/71XX1/7';
const score2 = '12324/XX9/247/X9/0';
const score3 = '6/54X6/367/X9/278/X';

function calculateScore(input) {
  const scoreArray = input.split('');

  const calculateSlashBonus = (previous, next) => 10 - previous + next;
  const calculateXBonus = (next, nextNext) => 10 + next + nextNext;
  const isFinalRound = index => index > 15;

  const scoreMap = scoreArray.map((score, index, array) => {
    const previousPrevious = +array[index - 2];
    const previous = array[index - 1] === '/' ? 10 - previousPrevious : +array[index - 1];
    const next = array[index + 1] === 'X' ? 10 : +array[index + 1];
    const nextNext = array[index + 2] === '/' ? 10 - next : +array[index + 2];

    // Special final round
    if (isFinalRound(index)) {
      if (score === '/') return 10 - previous;
      if (score === 'X') return 10;

      return +score;
    }

    if (score === '/') return calculateSlashBonus(previous, next);
    if (score === 'X') return calculateXBonus(next, nextNext);

    return +score;
  });

  const scoreTotal = scoreMap.reduce((prev, curr) => prev + curr);

  return scoreTotal;
}

console.log(`Score 1: ${calculateScore(score1)}`);
console.log(`Score 2: ${calculateScore(score2)}`);
console.log(`Score 3: ${calculateScore(score3)}`);