addEventListener("select", (event) => {});

onselect = (event) => {};

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

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.onselect = logSelection;