code: function string(rule, value, callback, source, options) {
    const errors = [];
    const validate = rule.required || (!rule.required && source.hasOwnProperty(rule.field));
    if (validate) {
      if (isEmptyValue(value, 'string') && !rule.required) {
        return callback();
      }
      rules.required(rule, value, source, errors, options, 'string');
      if (!isEmptyValue(value, 'string')) {
        rules.type(rule, value, source, errors, options);
        rules.range(rule, value, source, errors, options);
        rules.pattern(rule, value, source, errors, options);
        if (rule.whitespace === true) {
          rules.whitespace(rule, value, source, errors, options);
        }
      }
    }
    callback(errors);
  }
docs: Performs validation for string types.
  
  @param rule The validation rule.
  @param value The value of the field on the source object.
  @param callback The callback function.
  @param source The source object being validated.
  @param options The validation options.
  @param options.messages The validation messages.