plur.js

plur.js

const getItemAt = (list, index) =>
  list[Math.min(list.length - 1, Math.max(0, index))];

const plur = (...counts) => (strings, ...values) => {
  let i = 0;

  return strings
    .reduce((result, string, index) => {
      const value = values[index];

      return Array.isArray(value)
        ? [...result, string, getItemAt(value, getItemAt(counts, i++) - 1)]
        : typeof value === 'function'
        ? [...result, string, value(getItemAt(counts, i++))]
        : [...result, string, value];
    }, [])
    .join('');
};

export default plur;

plur.spec.js

import plur from './plur';

const sub2 = ['singular', 'plural'];
const sub3 = ['one', 'two', 'many'];
const value = 10;
const fSub = n => (n % 2 === 0 ? 'even' : 'odd');

test('should ignore value', () => {
  const actual = plur(1)`${value}`;

  expect(actual).toEqual('10');
});

test('should replace option w/ first', () => {
  const actual = plur(1)`${sub3}`;

  expect(actual).toEqual('one');
});

test('should replace option w/ middle', () => {
  const actual = plur(2)`${sub3}`;

  expect(actual).toEqual('two');
});

test('should replace w/ last', () => {
  const actual = plur(3)`${sub3}`;

  expect(actual).toEqual('many');
});

test('should replace w/ first when under', () => {
  const actual = plur(-10)`${sub3}`;

  expect(actual).toEqual('one');
});

test('should replace w/ last when over', () => {
  const actual = plur(10)`${sub3}`;

  expect(actual).toEqual('many');
});

test('should replace using function', () => {
  const actual = plur(2)`${fSub}`;

  expect(actual).toEqual('even');
});

test('should replace multiple times', () => {
  const actual = plur(1, 2, 3)`${sub2}, ${value}, ${sub3}, ${fSub}`;

  expect(actual).toEqual('singular, 10, two, odd');
});

test('should replace using previous count', () => {
  const actual = plur(1)`${sub2}, ${value}, ${sub3}`;

  expect(actual).toEqual('singular, 10, one');
});