@x-govuk/govuk-eleventy-plugin/src/filters/navigation.js

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import { sortCollection } from './collection.js'

/**
 * Flatten Eleventy navigation data
 *
 * @param {Array} eleventyNavigation - Eleventy navigation data
 * @returns {string} Flattened Eleventy navigation data
 */
function flattenNavigation(eleventyNavigation) {
  const navigationData = sortCollection(eleventyNavigation, true)

  let flattened = []
  for (const item of navigationData) {
    flattened.push(item)
    if (item.children && item.children.length > 0) {
      flattened = flattened.concat(flattenNavigation(item.children))
    }
  }

  return flattened
}

/**
 * Get next item in Eleventy navigation data
 *
 * @param {Array} eleventyNavigation - Eleventy navigation data
 * @param {string} pageUrl - URL of current page
 * @returns {object} Eleventy navigation data object
 */
export function getNextNavigationItem(eleventyNavigation, pageUrl) {
  const flatNavigation = flattenNavigation(eleventyNavigation)
  const currentIndex = flatNavigation.findIndex((item) => item.url === pageUrl)

  if (currentIndex >= 0 && currentIndex < flatNavigation.length - 1) {
    return flatNavigation[currentIndex + 1]
  }

  return null
}

/**
 * Get previous item in Eleventy navigation data
 *
 * @param {Array} eleventyNavigation - Eleventy navigation data
 * @param {string} pageUrl - URL of current page
 * @returns {object} Eleventy navigation data object
 */
export function getPreviousNavigationItem(eleventyNavigation, pageUrl) {
  const flatNavigation = flattenNavigation(eleventyNavigation)
  const currentIndex = flatNavigation.findIndex((item) => item.url === pageUrl)

  if (currentIndex > 0) {
    return flatNavigation[currentIndex - 1]
  }

  return null
}