refactor(www): more refactors for get-prev-and-next (#25627)

* more refactors for get-prev-and-next to hopefully make things better

* re-add condition

* fixes

www/src/utils/get-prev-and-next.js

@@ -1,7 +1,10 @@
+const _ = require(`lodash`)
 const { loadYaml } = require(`./load-yaml`)
-const docLinks = loadYaml(`src/data/sidebars/doc-links.yaml`)
-const tutorialLinks = loadYaml(`src/data/sidebars/tutorial-links.yaml`)
-const contributingLinks = loadYaml(`src/data/sidebars/contributing-links.yaml`)
+const navLinks = {
+  docs: loadYaml(`src/data/sidebars/doc-links.yaml`),
+  tutorial: loadYaml(`src/data/sidebars/tutorial-links.yaml`),
+  contributing: loadYaml(`src/data/sidebars/contributing-links.yaml`),
+}
 
 // flatten sidebar links trees for easier next/prev link calculation
 function flattenList(itemList) {
@@ -12,36 +15,30 @@ function flattenList(itemList) {
   }, [])
 }
 
-function flattenFilterList(itemList) {
-  const flattened = flattenList(itemList)
-  return flattened.filter(item => item.link && !item.link.includes(`#`))
+function normalize(slug) {
+  return slug.endsWith(`/`) ? slug : `${slug}/`
 }
 
-const flattenedNavs = {
-  docs: flattenFilterList(docLinks[0].items),
-  tutorial: flattenFilterList(tutorialLinks[0].items),
-  contributing: flattenFilterList(contributingLinks[0].items),
-}
+const flattenedNavs = _.mapValues(navLinks, navList => {
+  const flattened = flattenList(navList[0].items)
+  return flattened.filter(item => item.link && !item.link.includes(`#`))
+})
 
-function findDoc(doc) {
-  if (!doc.link) return null
-  return (
-    doc.link === this.link ||
-    doc.link === this.link.substring(0, this.link.length - 1) // deal with stubs
+const navIndicesBySlug = _.mapValues(flattenedNavs, navList =>
+  Object.fromEntries(
+    navList.map((item, index) => [normalize(item.link), index])
   )
-}
+)
 
 function getPrevAndNext(slug) {
   const section = slug.split(`/`)[1]
   const sectionNav = flattenedNavs[section]
   if (!sectionNav) return null
-  const index = sectionNav.findIndex(findDoc, { link: slug })
-  if (index < 0) {
-    return null
-  }
+  const index = navIndicesBySlug[section][normalize(slug)]
+  if (_.isNil(index)) return null
   return {
-    prev: index === 0 ? null : sectionNav[index - 1],
-    next: index === sectionNav.length - 1 ? null : sectionNav[index + 1],
+    prev: sectionNav[index - 1] || null,
+    next: sectionNav[index + 1] || null,
   }
 }