Server side form validator component for Express

routes.js

function isEmpty(value) {
  var valid = true;
  if(!value || value.trim().length == 0) {
    valid = false;
  }
  return valid;
}

router.post('/edit', (req, res) => {
  var v = new Validator(req, res);
  v.add('subject', [{ fn: isEmpty, message: 'Enter the subject' }]);
  v.add('body', [{ fn: isEmpty, message: 'Enter the question'	}]);
  if(v.validate()) {
    res.redirect('/');
  } else {
    res.render('template');
  }
});

Validator.js

function Validator(req, res) {
  this.req = req;
  this.res = res;
  this.validators = [];
  this.errors = [];
}

Validator.prototype.add = function(name, rules) {
  this.validators.push({
    name: name,
    rules: rules
  });
};

Validator.prototype.getErrors = function() {
	return this.errors;
};

Validator.prototype.getFormattedErrors = function() {
	return this.errors.map(this.formatError);
};

Validator.prototype.exposeInlineErrors = function() {
  this.res.locals.inlineErrors = {};
  this.errors.forEach(error => {
    const { name, message } = error;
    this.res.locals.inlineErrors[name] = { text: message };
  });
};

Validator.prototype.exposeErrorSummaryItems = function() {
  this.res.locals.errorSummary = {
    items: this.getFormattedErrors()
  };
};

Validator.prototype.formatError = function(error) {
	return {
    text: error.message,
		href: '#' + error.name
	};
};

Validator.prototype.getError = function(name) {
	return this.getErrors().filter(error => error.name == name).map(this.formatError)[0];
};

Validator.prototype.validate = function() {
  const body = this.req.body;
  const errors = [];

  for( let validator of this.validators ) {

    const { name, rules } = validator;
    const value = body[ name ];

    for( let rule of rules ){

      const { fn, params, message} = rule;
      const isValid = fn( value, params );

      if( !isValid ){
        errors.push( { name, message } );
        break;
      }
    }
  }

  this.errors = errors;
  this.exposeInlineErrors();
  this.exposeErrorSummaryItems();
  this.res.locals.hasValidationErrors = true;

  return errors.length == 0;
};

module.exports = Validator;

view.html

{% if errorSummary.items.length %}
  {{ govukErrorSummary({
    titleText: 'There is a problem',
    errorList: errorSummary.items
  }) }}
{% endif %}

{{ govukInput({
  label: { text: 'Subject'},
  id: 'subject',
  name: 'subject',
  errorMessage: inlineErrors.subject
}) }}

{{ govukTextarea({
  label: { text: 'Question' },
  id: 'body',
  name: 'body',
  errorMessage: inlineErrors.body
}) }}