WIP - poc(helpers): Generate validation errors

[ttbarnes]

⚠️ WIP

Initial example of a generic helper function that could be used in a NodeJS application to generate a validation error.

The resulting object can then be passed to a nunjucks template and consumed by the Error summary component.

This can be adapted to support mutliple validation errors. This is purely a simple example for a single validation error.

Having this in place would reduce duplication across different services and introduce a structure/patten for other helper functions.

For example, with the helper function in place, a service could simply import this and generate validation errors without having to write any specific error messaging logic:

// my-service/ui/server/controllers/example.js

import generateValidationError from '@govuk-frontend/helpers/generate-validation-error';

const REQUIRED_FIELD_ID = 'exampleField';

export const post = (req, res) => {
  if (!req.body[REQUIRED_FIELD_ID]) {
    const validationErrors = generateValidationError(REQUIRED_FIELD_ID, 'Field X is required');

    if (validationErrors) {
      return res.render('example.njk', {
        validationErrors,
        submittedValues: req.body,
      });
    }
  }

  return res.redirect('/next-page');
};

// my-service/ui/server/templates/example.njk

{% from 'govuk/components/error-summary/macro.njk' import govukErrorSummary %}
{% from 'govuk/components/radios/macro.njk' import govukRadios %}

{% block content %}
  {% if validationErrors.summary %}
    {{ govukErrorSummary({
      titleText: 'There is a problem",
      errorList: validationErrors.summary
    }) }}
  {% endif %}

  {{ govukRadios({
    idPrefix: 'exampleField',
    name: 'exampleField',
    errorMessage: validationErrors.errorList.exampleField and {
      text: validationErrors.errorList.exampleField.text
    }
  }) }}
{% endmacro %}