code: function customizer(objValue, srcValue) {
  	if (isUndefined(objValue) && !isUndefined(srcValue)) return srcValue
  	if (isArray(objValue) && isArray(srcValue)) return srcValue
  	if (isRegExp(objValue) || isRegExp(srcValue)) return srcValue
  	if (isObject(objValue) || isObject(srcValue)) return mergeWith(objValue, srcValue, customizer)
  }
docs: Customizer method to merge sources