pluralize

Returns the singular or plural form of the word based on the input number, using an optional dictionary if supplied.

• Use a closure to define a function that pluralizes the given word based on the value of num.
• If num is either -1 or 1, return the singular form of the word.
• If num is any other number, return the plural form.
• Omit the third argument, plural, to use the default of the singular word + s, or supply a custom pluralized word when necessary.
• If the first argument is an object, return a function which can use the supplied dictionary to resolve the correct plural form of the word.

const pluralize = (val, word, plural = word + 's') => {
  const _pluralize = (num, word, plural = word + 's') =>
    [1, -1].includes(Number(num)) ? word : plural;
  if (typeof val === 'object')
    return (num, word) => _pluralize(num, word, val[word]);
  return _pluralize(val, word, plural);
};

pluralize(0, 'apple'); // 'apples'
pluralize(1, 'apple'); // 'apple'
pluralize(2, 'apple'); // 'apples'
pluralize(2, 'person', 'people'); // 'people'

const PLURALS = {
  person: 'people',
  radius: 'radii'
};
const autoPluralize = pluralize(PLURALS);
autoPluralize(2, 'person'); // 'people'