code: function parseOptions(options) {
    let separator
    let transformer
  
    if (2 === options.length) {
      [separator, transformer] = options
      if (defaultTransformers[transformer]) {
        transformer = defaultTransformers[transformer] /* Don't validate */
        validate({ separator })
      } else {
        validate({ separator, transformer })
      }
    } else if (1 === options.length) {
      const option = options[0]
      if (false === option || 'function' === typeof option) {
        transformer = option /* Don't validate */
      } else if (defaultTransformers[option]) {
        transformer = defaultTransformers[option] /* Don't validate */
      } else {
        separator = option
        validate({ separator })
      }
    }
  
    return { separator, transformer }
  }
docs: Check and return validated options