classnames.js

function classnames (...classes) {
  const arr = []

  for (const name of classes) {
    if (typeof name === 'string') {
      arr.push(name)
    }
    else if (typeof name === 'object' && name !== null) {
      for (let [key, value] of Object.entries(name)) {
        if (value) arr.push(key)
      }
    }
  }

  return arr.join(' ')
}