mdn select event

function logSelection(event) {
  const log = document.getElementById('log');
  const selection = event.target.value.substring(event.target.selectionStart, event.target.selectionEnd);
  log.textContent = `You selected: ${selection}`;
}

const input = document.querySelector('input');
input.addEventListener('select', logSelection);

<input value="Try selecting some text in this element.">
<p id="log"></p>