Device Font Size ~ bjørn.dev

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/**
 * Returns a tuple with size and percentage
 * (relation to default font-size of 16px)
 *
 * @returns {array} - font-size and percentage
 */
function getUserFontSize() {
  const element = document.createElement('div');
  element.style.fontSize = 'initial';
  document.body.appendChild(element);
  const info = window
    .getComputedStyle(element)
    .getPropertyValue('font-size');
  element.remove();

  const fontSize = parseInt(info, 10);
  const percentage = (fontSize / 16) * 100;

  return [fontSize, percentage];
}