code: function getMappingType(mapping) {
    if (is.array(mapping)) {
      return every(isMixed, mapping) ? 'mixed' : 'array';
    }
    if (is.object(mapping)) return 'map';
    return 'unknown';
  }
docs: Determine the type of the option passed to `#map`
  
  @api private
  @param {Object|Object[]} mapping
  @return {String} mappingType