classnames.js

import { map, is, filter, keys, compose, identity, join } from 'ramda'

const processObj = compose(classnames, keys, filter(identity))

const processArr = compose(join(' '), filter(identity), map(classnames))

function classnames (arg) {
  switch (true) {
    case is(String, arg): return arg
    case is(Array, arg): return processArr(arg)
    case is(Object, arg): return processObj(arg)
    default: return null
  }
}

const cn = (...args) => classnames(args)

describe.only('classnames', function () {
  it('merges simple strings', function () {
    expect(cn('first', 'second')).to.eq('first second')
  })
  it('merges array simple strings', function () {
    expect(cn(['first', 'second'])).to.eq('first second')
  })
  it('merges object of strings/predicates', function () {
    expect(cn({ 'first': true, 'second': false })).to.eq('first')
  })
  it('merges damn complex case', function () {
    expect(cn('first', { 'second': true }, [{ 'third': false, 'fourth': true }])).to.eq('first second fourth')
  })
})