parseInt() reloaded solution https://www.codewars.com/kata/525c7c5ab6aecef16e0001a5/train/javascript

parseInt.js

// >= 0, <= 1_000_000
// problem suggests that string can only contain 'million'
// or less -> million can occure only once and it's the whole string
const parseInt = (string) =>
  string.includes('million')
  ? 1000000 : parseMillion(string);

// < 1_000_000
// if we have 'thousand' word in given string,
// split on it and solve parse left, multiply it by 1000
// solve right part and sum
const parseMillion = (string) => {
  if (string.includes('thousand')) {
    let [left, right] = string.split('thousand');
    return parseThousand(left) * 1000 + parseThousand(right);
  }
  // if string has no 'thousand', it is less than 1000
  return parseThousand(string);
};

// < 1_000
// look parseMillion, the same logic
const parseThousand = (string) => {
  if (string.includes('hundred')) {
    let [left, right] = string.split('hundred');
    return parseHundred(left) * 100 + parseHundred(right);
  }
  return parseHundred(string);
};

// < 100
// split string on space and '-'
// delete empty words and 'and' words
// transalte words into integers and find its sum
const parseHundred = (string) => {
  let words = string.split(/[\s-]/)
          .filter(w => w !== '')
          .filter(w => w !== 'and');
  return words.map(w => wordToInt[w]).reduce((x, acc) => acc + x, 0);
};

// map words to significant integers
const wordToInt = {
  zero: 0, one: 1, two: 2, three: 3, four: 4,
  five: 5, six: 6, seven: 7, eight: 8, nine: 9,
  ten: 10, eleven: 11, twelve: 12, thirteen: 13, fourteen: 14,
  fifteen: 15, sixteen: 16, seventeen: 17, eighteen: 18, nineteen: 19,
  twenty: 20, thirty: 30, forty: 40, fifty: 50, sixty: 60, seventy: 70,
  eighty: 80, ninety: 90,
  hundred: 100, thousand: 1000, million: 1000000 // these are unnecessary
};