refactor(build): modularize JS code

- replace gulp with rollup
- remove JS output from repo

_javascript/modules/components/smooth-scroll.js

@@ -0,0 +1,109 @@
+/**
+ Safari doesn't support CSS `scroll-behavior: smooth`,
+ so here is a compatible solution for all browser to smooth scrolling
+
+ See: <https://css-tricks.com/snippets/jquery/smooth-scrolling/>
+
+ Warning: It must be called after all `<a>` tags (e.g., the dynamic TOC) are ready.
+ */
+import ScrollHelper from './utils/scroll-helper';
+
+export function smoothScroll() {
+  const $topbarTitle = $('#topbar-title');
+  const REM = 16; // in pixels
+  const ATTR_SCROLL_FOCUS = 'scroll-focus';
+  const SCOPE = "a[href*='#']:not([href='#']):not([href='#0'])";
+
+  $(SCOPE).on('click', function (event) {
+    if (
+      this.pathname.replace(/^\//, '') !== location.pathname.replace(/^\//, '')
+    ) {
+      return;
+    }
+
+    if (location.hostname !== this.hostname) {
+      return;
+    }
+
+    const hash = decodeURI(this.hash);
+    let toFootnoteRef = RegExp(/^#fnref:/).test(hash);
+    let toFootnote = toFootnoteRef ? false : RegExp(/^#fn:/).test(hash);
+    let selector = '#' + $.escapeSelector(hash.substring(1));
+    let $target = $(selector);
+
+    let isMobileViews = $topbarTitle.is(':visible');
+    let isPortrait = $(window).width() < $(window).height();
+
+    if (typeof $target === 'undefined') {
+      return;
+    }
+
+    event.preventDefault();
+
+    if (history.pushState) {
+      /* add hash to URL */
+      history.pushState(null, null, hash);
+    }
+
+    let curOffset = $(window).scrollTop();
+    let destOffset = ($target.offset().top -= REM / 2);
+
+    if (destOffset < curOffset) {
+      // scroll up
+      ScrollHelper.hideTopbar();
+      ScrollHelper.addScrollUpTask();
+
+      if (isMobileViews && isPortrait) {
+        destOffset -= ScrollHelper.getTopbarHeight();
+      }
+    } else {
+      // scroll down
+      if (isMobileViews && isPortrait) {
+        destOffset -= ScrollHelper.getTopbarHeight();
+      }
+    }
+
+    $('html').animate(
+      {
+        scrollTop: destOffset
+      },
+      500,
+      () => {
+        $target.trigger('focus');
+
+        /* clean up old scroll mark */
+        const $scroll_focus = $(`[${ATTR_SCROLL_FOCUS}=true]`);
+        if ($scroll_focus.length) {
+          $scroll_focus.attr(ATTR_SCROLL_FOCUS, 'false');
+        }
+
+        /* Clean :target links */
+        const $target_links = $(':target');
+        if ($target_links.length) {
+          /* element that visited by the URL with hash */
+          $target_links.attr(ATTR_SCROLL_FOCUS, 'false');
+        }
+
+        /* set scroll mark to footnotes */
+        if (toFootnote || toFootnoteRef) {
+          $target.attr(ATTR_SCROLL_FOCUS, 'true');
+        }
+
+        if ($target.is(':focus')) {
+          /* Checking if the target was focused */
+          return false;
+        } else {
+          $target.attr(
+            'tabindex',
+            '-1'
+          ); /* Adding tabindex for elements not focusable */
+          $target.trigger('focus'); /* Set focus again */
+        }
+
+        if (ScrollHelper.hasScrollUpTask()) {
+          ScrollHelper.popScrollUpTask();
+        }
+      }
+    );
+  }); /* click() */
+}