Unit word form, word ending, unit suffix (for Russian)

get-unit-form.js

// type getUnitForm = ([string, string, string]) => (number) => string
export const getUnitForm = ([nominative, genitive, plural]) => (num) => {
    num = Math.abs(num) % 100

    if (num >= 11 && num <= 19) return plural

    switch (num % 10) {
        case (1): return nominative
        case (2):
        case (3):
        case (4): return genitive
        default: return plural
    }
}

get-unit-form.test.js

import {getUnitForm} from './get-unit-form'

describe('getUnitForm', () => {
  const fn = getUnitForm(['зал', 'зала', 'залов'])

  it('should return "зал" for 1, 21, 31, 101', () => {
    const expected = 'зал'

    expect(fn(1)).toBe(expected)
    expect(fn(21)).toBe(expected)
    expect(fn(31)).toBe(expected)
    expect(fn(101)).toBe(expected)
  })

  it('should return "зала" for 2, 22, 33, 104', () => {
    const expected = 'зала'

    expect(fn(2)).toBe(expected)
    expect(fn(22)).toBe(expected)
    expect(fn(33)).toBe(expected)
    expect(fn(104)).toBe(expected)
  })

  it('should return "залов" for 0, 5, 11, 12, 26, 37, 100, 111, 114', () => {
    const expected = 'залов'

    expect(fn(0)).toBe(expected)
    expect(fn(5)).toBe(expected)
    expect(fn(11)).toBe(expected)
    expect(fn(12)).toBe(expected)
    expect(fn(26)).toBe(expected)
    expect(fn(37)).toBe(expected)
    expect(fn(100)).toBe(expected)
    expect(fn(111)).toBe(expected)
    expect(fn(114)).toBe(expected)
  })
})