Optimize main.js (#20093)

This Pull Request optimizes the functions used in
`/docs/_spec/public/scripts/main.js`.

## Changes made:

1. `currentChapter()` function was simplified using `split()` and
`pop()` instead of `lastIndexOf()` and `substring()`.
2. Used template literals for string interpolation.
3. Simplified the `heading` function by reducing repetitive code and
improving readability.
4. Changed `.removeClass()` and `.addClass()` to `.toggleClass()` for
toggling the class based on condition.
5. General cleanup and optimization for better readability and
performance.
[Cherry-picked 6d29951]

docs/_spec/public/scripts/main.js

@@ -1,34 +1,29 @@
 function currentChapter() {
-  var path = document.location.pathname;
-  var idx  = path.lastIndexOf("/") + 1;
-  var chap = path.substring(idx, idx + 2);
-  return parseInt(chap, 10);
+  return parseInt(document.location.pathname.split('/').pop().substr(0, 2), 10);
 }
 
 function heading(i, heading, $heading) {
-  var currentLevel = parseInt(heading.tagName.substring(1));
-  var result = "";
+  const currentLevel = parseInt(heading.tagName.substring(1));
+
   if (currentLevel === this.headerLevel) {
-    this.headerCounts[this.headerLevel] += 1;
-    return "" + this.headerCounts[this.headerLevel] + " " + $heading.text();
+    this.headerCounts[this.headerLevel]++;
   } else if (currentLevel < this.headerLevel) {
-    while(currentLevel < this.headerLevel) {
+    while (currentLevel < this.headerLevel) {
       this.headerCounts[this.headerLevel] = 1;
-      this.headerLevel -= 1;
+      this.headerLevel--;
     }
-    this.headerCounts[this.headerLevel] += 1;
-    return "" + this.headerCounts[this.headerLevel]+ " " + $heading.text();
+    this.headerCounts[this.headerLevel]++;
   } else {
-    while(currentLevel > this.headerLevel) {
-      this.headerLevel += 1;
+    while (currentLevel > this.headerLevel) {
+      this.headerLevel++;
       this.headerCounts[this.headerLevel] = 1;
     }
-    return "" + this.headerCounts[this.headerLevel]+ " " + $heading.text();
   }
+  return `${this.headerCounts[this.headerLevel]} ${$heading.text()}`;
 }
 
 // ignore when using wkhtmltopdf, or it won't work...
-if(window.jekyllEnv !== 'spec-pdf') {
+if (window.jekyllEnv !== 'spec-pdf') {
   $('#toc').toc(
     {
       'selectors': 'h1,h2,h3',
@@ -64,8 +59,6 @@ document.addEventListener("DOMContentLoaded", function() {
 });
 
 $("#chapters a").each(function (index) {
- if (document.location.pathname.endsWith($(this).attr("href")))
-   $(this).addClass("chapter-active");
- else
-   $(this).removeClass("chapter-active");
+  const href = $(this).attr("href");
+  $(this).toggleClass("chapter-active", document.location.pathname.endsWith(href));
 });