Block Wipe Rotator

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Block Wipe Rotator: a solid coloured block sweeps over the word in your
    // sentence, the word changes underneath it, and the block slides off the
    // far side to leave the next word behind.
    // Usage: <h1>Make it <span data-block-wipe-rotator="bold, loud, yours">bold</span></h1>
    // Optional: data-hold (ms per word, default 1800), data-duration (ms per
    // wipe, default 1000), data-color (the block, default the text colour),
    // data-direction ("right", the default, sweeps left to right; "left" sweeps
    // right to left).
    var DEFAULTS = { hold: 1800, duration: 1000 };

    var css =
      '.bwr{display:inline-block;position:relative;white-space:nowrap;vertical-align:baseline;text-align:left}' +
      '.bwr-block{position:absolute;left:-.06em;right:-.06em;top:.06em;bottom:.02em;background:currentColor;transform:scaleX(0);pointer-events:none}' +
      '.bwr-probe{position:absolute;left:0;top:0;visibility:hidden;white-space:nowrap}' +
      '.bwr-sr{position:absolute;width:1px;height:1px;overflow:hidden;clip:rect(0 0 0 0);white-space:nowrap}';
    var style = document.createElement('style');
    style.textContent = css;
    document.head.appendChild(style);

    function span(cls, parent, text) {
      var s = document.createElement('span');
      s.className = cls;
      if (text) s.textContent = text;
      parent.appendChild(s);
      return s;
    }

    function setup(el) {
      var words = (el.getAttribute('data-block-wipe-rotator') || el.textContent)
        .split(',').map(function (w) { return w.trim(); }).filter(Boolean);
      el.classList.add('bwr');
      el.textContent = '';
      span('bwr-sr', el, words.join(', '));
      var out = span('bwr-word', el, words[0]);
      var block = span('bwr-block', el);
      var probe = span('bwr-probe', el);
      [out, block, probe].forEach(function (n) { n.setAttribute('aria-hidden', 'true'); });
      if (el.dataset.color) block.style.background = el.dataset.color;
      return { el: el, words: words, out: out, block: block, probe: probe, i: 0, w: 0, busy: false, visible: false, started: false };
    }

    function measure(s, text) {
      s.probe.textContent = text;
      return s.probe.offsetWidth;
    }

    function fit(s) {
      if (s.busy) return;
      s.w = measure(s, s.words[s.i]);
      s.el.style.width = s.w + 'px';
    }

    function swap(s, done) {
      var d = s.el.dataset;
      var t = +d.duration || DEFAULTS.duration;
      var fromLeft = d.direction !== 'left';
      s.i = (s.i + 1) % s.words.length;
      var to = s.words[s.i];
      s.busy = true;
      // 1. The block grows over the word from one side.
      anime.utils.set(s.block, { transformOrigin: fromLeft ? '0% 50%' : '100% 50%' });
      anime.animate(s.block, {
        scaleX: [0, 1], duration: t * 0.4, ease: 'inOut(3)',
        onComplete: function () {
          // 2. Hidden under the block, the word changes and the box resizes.
          s.out.textContent = to;
          var w1 = measure(s, to);
          anime.animate(s, {
            w: w1, duration: t * 0.2, ease: 'inOut(2)',
            onUpdate: function () { s.el.style.width = s.w.toFixed(2) + 'px'; },
            onComplete: function () {
              // 3. The block shrinks away towards the far side.
              anime.utils.set(s.block, { transformOrigin: fromLeft ? '100% 50%' : '0% 50%' });
              anime.animate(s.block, {
                scaleX: [1, 0], duration: t * 0.4, ease: 'inOut(3)',
                onComplete: function () { s.busy = false; done(); }
              });
            }
          });
        }
      });
    }

    // Wait, then run fn, but only once the word is on screen.
    function wait(s, ms, fn) {
      setTimeout(function check() {
        if (s.visible && !document.hidden) fn();
        else setTimeout(check, 300);
      }, ms);
    }

    function start(s) {
      fit(s);
      if (document.fonts) document.fonts.ready.then(function () { fit(s); });
      window.addEventListener('resize', function () { fit(s); });
      var hold = +s.el.dataset.hold || DEFAULTS.hold;
      (function loop() {
        wait(s, hold, function () { swap(s, loop); });
      })();
    }

    function init() {
      var reduced = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
      var io = new IntersectionObserver(function (entries) {
        entries.forEach(function (e) {
          var s = e.target._bwr;
          s.visible = e.isIntersecting;
          if (s.visible && !s.started) { s.started = true; start(s); }
        });
      }, { threshold: 0.3 });
      document.querySelectorAll('[data-block-wipe-rotator]').forEach(function (el) {
        if (el._bwr) return;
        el._bwr = setup(el);
        if (!reduced && el._bwr.words.length > 1) io.observe(el);
      });
    }

    if (document.readyState === 'loading') document.addEventListener('DOMContentLoaded', init);
    else init();
  })();
</script>

<h1>
  Make it
  <span data-block-wipe-rotator="bold, loud, yours">bold</span>
</h1>