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js/scrollSpy.js

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+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+/* eslint-disable prefer-arrow-callback */
+(function scrollSpy() {
+  const OFFSET = 10;
+  let timer;
+  let headingsCache;
+  const findHeadings = function findHeadings() {
+    return headingsCache || document.querySelectorAll('.toc-headings > li > a');
+  };
+  const onScroll = function onScroll() {
+    if (timer) {
+      // throttle
+      return;
+    }
+    timer = setTimeout(function() {
+      timer = null;
+      let activeNavFound = false;
+      const headings = findHeadings(); // toc nav anchors
+      /**
+       * On every call, try to find header right after  <-- next header
+       * the one whose content is on the current screen <-- highlight this
+       */
+      for (let i = 0; i < headings.length; i++) {
+        // headings[i] is current element
+        // if an element is already active, then current element is not active
+        // if no element is already active, then current element is active
+        let currNavActive = !activeNavFound;
+        /**
+         * Enter the following check up only when an active nav header is not yet found
+         * Then, check the bounding rectangle of the next header
+         * The headers that are scrolled passed will have negative bounding rect top
+         * So the first one with positive bounding rect top will be the nearest next header
+         */
+        if (currNavActive && i < headings.length - 1) {
+          const next = headings[i + 1].href.split('#')[1];
+          const nextHeader = document.getElementById(next);
+          const top = nextHeader.getBoundingClientRect().top;
+          currNavActive = top > OFFSET;
+        }
+        /**
+         * Stop searching once a first such header is found,
+         * this makes sure the highlighted header is the most current one
+         */
+        if (currNavActive) {
+          activeNavFound = true;
+          headings[i].classList.add('active');
+        } else {
+          headings[i].classList.remove('active');
+        }
+      }
+    }, 100);
+  };
+  document.addEventListener('scroll', onScroll);
+  document.addEventListener('resize', onScroll);
+  document.addEventListener('DOMContentLoaded', function() {
+    // Cache the headings once the page has fully loaded.
+    headingsCache = findHeadings();
+    onScroll();
+  });
+})();