Underline Wipe Swap

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/bundles/anime.umd.min.js"></script>
<script>
  (function () {
    // Underline Wipe Swap: the word in your sentence sits on an underline. The
    // underline sweeps away, the word changes as the line is gone, and the
    // underline sweeps back in beneath the new word.
    // Usage: <h1>Write <span data-underline-wipe-swap="clearly, boldly, freely">clearly</span></h1>
    // Optional: data-hold (ms per word, default 1800), data-duration (ms per
    // swap, default 1100), data-color (the line, default the word's colour),
    // data-thickness (the line, in em, default 0.08).
    var DEFAULTS = { hold: 1800, duration: 1100, thickness: 0.08 };

    var css =
      '.uws{display:inline-block;position:relative;white-space:nowrap;vertical-align:baseline;text-align:left}' +
      '.uws-word{display:inline-block}' +
      '.uws-line{position:absolute;left:0;right:0;bottom:-.1em;background:currentColor;border-radius:1em;transform-origin:0 50%;pointer-events:none}' +
      '.uws-probe{position:absolute;left:0;top:0;visibility:hidden;white-space:nowrap}' +
      '.uws-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-underline-wipe-swap') || el.textContent)
        .split(',').map(function (w) { return w.trim(); }).filter(Boolean);
      var d = el.dataset;
      el.classList.add('uws');
      el.textContent = '';
      span('uws-sr', el, words.join(', '));
      var out = span('uws-word', el, words[0]);
      var line = span('uws-line', el);
      var probe = span('uws-probe', el);
      [out, line, probe].forEach(function (n) { n.setAttribute('aria-hidden', 'true'); });
      line.style.height = (+d.thickness || DEFAULTS.thickness) + 'em';
      if (d.color) line.style.background = d.color;
      return { el: el, words: words, out: out, line: line, 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 t = +s.el.dataset.duration || DEFAULTS.duration;
      s.i = (s.i + 1) % s.words.length;
      var to = s.words[s.i];
      s.busy = true;
      // 1. The line sweeps out to the right, and the word sinks and fades a
      //    little behind it.
      anime.utils.set(s.line, { transformOrigin: '100% 50%' });
      anime.animate(s.line, { scaleX: [1, 0], duration: t * 0.38, ease: 'inOut(3)' });
      anime.animate(s.out, {
        opacity: [1, 0], y: ['0em', '0.18em'], duration: t * 0.3, delay: t * 0.1, ease: 'in(2)',
        onComplete: function () {
          // 2. With the line gone the word changes and the box resizes.
          s.out.textContent = to;
          anime.animate(s, {
            w: measure(s, to), duration: t * 0.3, ease: 'inOut(2)',
            onUpdate: function () { s.el.style.width = s.w.toFixed(2) + 'px'; }
          });
          anime.animate(s.out, { opacity: [0, 1], y: ['-0.18em', '0em'], duration: t * 0.35, ease: 'out(3)' });
          // 3. The line sweeps back in from the left under the new word.
          anime.utils.set(s.line, { transformOrigin: '0% 50%' });
          anime.animate(s.line, {
            scaleX: [0, 1], duration: t * 0.42, delay: t * 0.18, ease: 'out(4)',
            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._uws;
          s.visible = e.isIntersecting;
          if (s.visible && !s.started) { s.started = true; start(s); }
        });
      }, { threshold: 0.3 });
      document.querySelectorAll('[data-underline-wipe-swap]').forEach(function (el) {
        if (el._uws) return;
        el._uws = setup(el);
        if (!reduced && el._uws.words.length > 1) io.observe(el);
      });
    }

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

<h1>
  Write
  <span data-underline-wipe-swap="clearly, boldly, freely">clearly</span>
</h1>