(arg → string)
return string
- check and get argument
function (arg) {
  var result
  switch (arg) {
    case 'on':
      result = arg
      break
    case 'once':
      result = arg
      break
    case 'class':
      result = arg
      break
    case 'style':
      result = arg
      break
    case 'enter':
      result = arg
      break
    case 'leave':
      result = arg
      break
    case 'animate':
      result = arg
      break
    case 'parent':
      result = arg
      break
    default:
      console.warn('[in-view] argument ${arg} undefined')
  }
  return result
}