/**
 * Helper function to build an error message for a missing field
 *
 * @param {string} val The dotted path to the missing field
 * @return {string} The error message
 */
const errorTemplate = val => `Mandatory tracking value "${val}" is missing`;

/**
 * Validates tracking calls
 *
 * @param {Object} trackingData The data that needs to be tracked
 * @return {Array.<Error>}
 */
const validateTrackingData = ({ track, page, identify }) => {
  const errors = [];

  // Validate mandatory fields for Track API
  if (track) {
    if (!track.event) {
      errors.push('track.event');
    }

    if (!track.properties || !track.properties.category) {
      errors.push('track.properties.category');
    }
  }

  // Validate mandatory fields for Page API
  if (page) {
    if (!page.name) {
      errors.push('page.name');
    }

    if (!page.properties || !page.properties.url) {
      errors.push('page.properties.url');
    }

    if (!page.properties || !page.properties.title) {
      errors.push('page.properties.title');
    }

    if (!page.properties || !page.properties.path) {
      errors.push('page.properties.path');
    }
  }

  // Validate mandatory fields for Identify API
  if (identify) {
    if (!identify.userId) {
      errors.push('identify.userId');
    }
  }

  return errors.map(e => new Error(errorTemplate(e)));
};

export { validateTrackingData };