Beer song code kata

beer-song.js

module.exports = function () {
  this.verse = (n) => {
    const otw = 'on the wall'
    const numNow = n === 0 ? 'no more' : n
    const itOne = numNow < 2 ? 'it' : 'one'
    const numAfter = ((n - 1) < 0) ? 99 : ((n - 1) === 0) ? 'no more' : n - 1

    return [
      `${cap(numNow)} ${bottle(n)} ${otw}, `,
      `${numNow} ${bottle(n)}.`,
      `\n`,
      n > 0 ? `Take ${itOne} down and pass it around, ` : `Go to the store and buy some more, `,
      `${numAfter} ${bottle(numAfter)} ${otw}.`,
      `\n`
    ].join('')
  }

  this.sing = (curr, end) => {
    if (curr <= end) {
      return new Error('invalid curr/end verse')
    }

    const res = []
    end = end || 0

    while (curr >= end) {
      res.push(this.verse(curr))
      curr--
    }
    return res.join('\n')
  }
}

bottle = (n) => {
  const s = n !== 1 ? 's' : ''
  return `bottle${s} of beer`
}

cap = (str) => {
  str = str.toString()
  return str.substr(0, 1).toUpperCase() + str.substr(1)
}