Char Fade Up

export function charFadeUp(el) {
  const _reducedMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches
  const _duration = _reducedMotion ? 0.01 : 0.6
  const _stagger = _reducedMotion ? 0 : 0.02
  const _delay = _reducedMotion ? 0 : 0

  const split = splitChars(el)
  const easing = 'linear(0.0000,0.0987,0.1899,0.2740,0.3513,0.4224,0.4874,0.5467,0.6008,0.6499,0.6943,0.7343,0.7703,0.8025,0.8311,0.8566,0.8790,0.8987,0.9159,0.9308,0.9437,0.9546,0.9639,0.9717,0.9781,0.9834,0.9877,0.9910,0.9937,0.9957,0.9972,0.9982,0.9990,0.9994,0.9997,0.9999,1.0000,1.0000,1.0000,1.0000)'
  const anims = split.chars.map((c, i) =>
    c.animate(
      [
        { transform: 'translateY(100%)', opacity: 0 },
        { transform: 'none', opacity: 1 },
      ],
      {
        duration: _duration * 1000,
        delay: (_delay + i * _stagger) * 1000,
        easing,
        fill: 'backwards',
      },
    ),
  )
  return () => {
    anims.forEach((a) => a.cancel())
    split.revert()
  }
}

function splitChars(el) {
  const text = el.textContent ?? ''

  if (el.children.length > 0) {
    return { chars: [el], words: [el], revert: () => {} }
  }

  const escape = (s) => s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;')

  const segmenter =
    typeof Intl !== 'undefined' && 'Segmenter' in Intl
      ? new Intl.Segmenter(undefined, { granularity: 'grapheme' })
      : null
  const toChars = (s) => (segmenter ? Array.from(segmenter.segment(s), (seg) => seg.segment) : Array.from(s))

  const tokens = text.match(/\S+|\s+/g) ?? []
  el.setAttribute('aria-label', text)
  el.innerHTML = tokens
    .map((token) => {
      if (/^\s+$/.test(token)) return token
      const chars = toChars(token)
        .map((c) => `<span class="k-char">${escape(c)}</span>`)
        .join('')
      return `<span class="k-word" aria-hidden="true">${chars}</span>`
    })
    .join('')

  return {
    chars: [...el.querySelectorAll('.k-char')],
    words: [...el.querySelectorAll('.k-word')],
    revert: () => {
      el.textContent = text
      el.removeAttribute('aria-label')
    },
  }
}

/* CSS */
.k-word {
  display: inline-block;
  white-space: nowrap;
}

.k-char {
  display: inline-block;
}