Add callback to reuse fetched nav items

Fixes #15.

package.json

@@ -1,6 +1,6 @@
 {
     "name": "@fwdekker/template",
-    "version": "2.3.1",
+    "version": "2.4.0",
     "description": "The base template for pages on fwdekker.com.",
     "author": "Felix W. Dekker",
     "license": "MIT",

src/main/js/main.js

@@ -47,9 +47,10 @@ const doAfterLoad = function(fun) {
  * Fetches entries asynchronously from the website's API.
  *
  * @param [highlightPath] {String} the path to highlight together with its parents
+ * @param [cb] {Function} the callback to execute on the fetched entries, to prevent the need to re-fetch elsewhere
  * @returns {HTMLElement} a base navigation element that will eventually be filled with contents
  */
-const nav = function(highlightPath = "") {
+const nav = function(highlightPath = "", cb = undefined) {
     const base = stringToHtml(
         `<ul><li><a href="https://fwdekker.com/">` +
         `<div class="logo"><img class="logo" src="https://fwdekker.com/favicon.png" alt="FWDekker" /></div>` +
@@ -61,7 +62,9 @@ const nav = function(highlightPath = "") {
     fetch("https://fwdekker.com/api/nav/")
         .then(it => it.json())
         .then(json => {
-            json.entries.forEach(entry => base.appendChild(stringToHtml(unpackEntry(entry, "/", highlightPath), "li")))
+            if (cb !== undefined) cb(json);
+
+            json.entries.forEach(entry => base.appendChild(stringToHtml(unpackEntry(entry, "/", highlightPath), "li")));
         })
         .catch(e => {
             console.error("Failed to fetch navigation elements", e);