Method for creating abstracted string generators

strings.js

/**
 * Returns a function that composes strings from ES6 template literals.
 * Based on http://stackoverflow.com/a/22619256/11577
 *
 * @usage formatter`${0}, ${1}!`.format('Hello', 'world') // -> 'Hello, world!'
 *
 * @param array<string> literals Array of string literals
 * @param array<mixed> substitutions Rest parameter of substitution values
 *
 * @return function
 */
exports.formatter = function formatter(literals, ...substitutions) {
  return {
    format: function format() {
      return literals.reduce((accumulator, part, i) => {
        let sub = arguments[substitutions[i - 1]]
        sub = Array.isArray(sub) ? combineMultiple(sub) : sub
        return accumulator + sub + part
      })
    }
  }
}

/**
 * Combine multiple values in an array in a standard way
 *
 * @usage combineMultiple(['Me']) // -> 'Me'
 * @usage combineMultiple(['Me', 'You']) // -> 'Me &amp; You'
 * @usage combineMultiple(['Me', 'You', 'Us']) // -> 'Me, You &amp; Us'
 *
 * @param array<mixed> Array of values
 *
 * @return string
 */
function combineMultiple(arr) {
  switch (arr.length) {
    case 0:
      return ''
    case 1:
      return `${arr[0]}` // always return a string
    case 2:
      return `${arr[0]} &amp; ${arr[1]}`
    default:
      let lastVal = arr.pop()
      return `${arr.join(', ')} &amp; ${lastVal}`
  }
}

// The idea is to create helpers (similar to `itemsAdded()` below) for creating
// the various formats of strings that will be needed in the activities
// interface. I haven’t worked out yet how this will mesh with the translation
// library.
/*

function itemsAdded() {
  return formatter`${0} added ${1} ${2} to ${3}`.format.call(null, ...arguments)
}

itemsAdded(['James Chou'], 42, 'photos', 'Pretty Sunsets')
// => "James Chou added 42 photos to Pretty Sunsets"

itemsAdded(['James Chou', 'Terry Liu'], 42, 'photos', 'Pretty Sunsets')
// => "James Chou &amp; Terry Liu added 42 photos to Pretty Sunsets"

itemsAdded(['James Chou', 'Terry Liu', 'Fan Zhang'], 42, 'photos', 'Pretty Sunsets')
// => "James Chou, Terry Liu &amp; Fan Zhang added 42 photos to Pretty Sunsets"

*/

// One way to use this for localiztion would be to create helpers based on
// locale. We could end up with a lot of functions, but it would give us exact
// control over the sentence structure for each language. But even my simple
// example below illustrates the complexity of localization. The word ‘photos’
// would need to be translated before it hits the helper the way I have the
// functions written.
/*

let helpers = {
  de: {
    itemsAdded: () => formatter`${0} hinzugefügt ${1} ${2} zu ${3}`.format.call(null, ...arguments)
  }
  en: {
    itemsAdded: () => formatter`${0} added ${1} ${2} to ${3}`.format.call(null, ...arguments)
  },
  es: {
    itemsAdded: () => formatter`${0} añadido ${1} ${2} a ${3}`.format.call(null, ...arguments)
  },
  fr: {
    itemsAdded: () => formatter`${0} a ajouté ${1} ${2} à ${3}`.format.call(null, ...arguments)
  }
}

const locale = store.getIn(['ui', 'locale'])
const action = 'itemsAdded'

const output = helpers[locale][action](['James Chou'], 42, 'photos', 'Pretty Sunsets')
// 'de' => James Chou hinzugefügt 42 Fotos zu Pretty Sunsets
// 'en' => James Chou added 42 photos to Pretty Sunsets
// 'es' => James Chou añadido 42 fotos a Pretty Sunsets
// 'fr' => James Chou a ajouté 42 photos à Pretty Sunsets

*/