optimalFontSize.js

const optimalFontSize = (word, r, fontFamily, fontWeight) => {
  const text = document.createElementNS('http://www.w3.org/2000/svg', 'text')
  text.textContent = word
  text.setAttributeNS(null, 'font-family', fontFamily)
  text.setAttributeNS(null, 'font-weight', fontWeight)
  const svg = document.createElementNS('http://www.w3.org/2000/svg', 'svg')
  svg.appendChild(text)
  document.body.appendChild(svg)
  let ok = 0
  let ng = 100
  for (let iter = 0; iter < 10; ++iter) {
    let m = (ok + ng) / 2
    text.setAttributeNS(null, 'font-size', m)
    const { width, height } = text.getBBox()
    const d = Math.sqrt(width ** 2 + height ** 2) / 2
    if (d <= r) {
      ok = m
    } else {
      ng = m
    }
  }
  document.body.removeChild(svg)
  return ok
}