index.js

// Section 1.
//   (a)
//     (i)
//       (A)
//         (I)

var lowerAlpha = require('lower-alpha')

function upperAlpha (i) {
  return lowerAlpha(i).toUpperCase()
}

var upperRoman = require('romanize')

function lowerRoman (i) {
  return upperRoman(i).toLowerCase()
}

function arabic (i) {
  return i.toString()
}

function compound (primary, secondary) {
  return function (element, series) {
    return (
      (series ? secondary(series) + '-' : '') +
      primary(element)
    )
  }
}

function inTheHole (primary, secondary) {
  var compounded = compound(primary, secondary)
  return function (element, series) {
    return '(' + compounded(element, series) + ')'
  }
}

var LEVEL_FORMATTERS = [null, compound(arabic, upperAlpha)]

var REPEATING_FORMATTERS = [
  inTheHole(lowerAlpha, arabic),
  inTheHole(lowerRoman, arabic),
  inTheHole(upperAlpha, arabic),
  inTheHole(upperRoman, arabic)
]

function formatterForLevel (level) {
  if (level < LEVEL_FORMATTERS.length) {
    return LEVEL_FORMATTERS[level]
  } else {
    var offset = level - LEVEL_FORMATTERS.length
    var index = offset % REPEATING_FORMATTERS.length
    return REPEATING_FORMATTERS[index]
  }
}

function renderComponent (component, level) {
  return formatterForLevel(level)(
    component.element.number,
    component.series.of > 1 ? component.series.number : null
  )
}

module.exports = function (numbering, abbreviated) {
  if (abbreviated) {
    var length = numbering.length
    return (
      renderComponent(numbering[length - 1], length) +
      (length === 1 ? '.' : '')
    )
  } else {
    return (
      'Section\u00a0' +
      numbering.reduce(function (number, component, i) {
        return number + renderComponent(component, i + 1)
      }, '')
    )
  }
}