Документ: свойство activeElement

<p>Select some text from one of the text areas below:</p>

<form>
  <textarea name="ta-example-one" id="ta-example-one" rows="7" cols="40">
This is Text Area One. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec tincidunt, lorem a porttitor molestie, odio nibh iaculis libero, et accumsan nunc orci eu dui.</textarea
  >
  <textarea name="ta-example-two" id="ta-example-two" rows="7" cols="40">
This is Text Area Two. Fusce ullamcorper, nisl ac porttitor adipiscing, urna orci egestas libero, ut accumsan orci lacus laoreet diam. Morbi sed euismod diam.</textarea
  >
</form>

<p>Active element ID: <em id="output-element"></em></p>
<p>Selected text: <em id="output-text"></em></p>

function onMouseUp(e) {
  const activeTextarea = document.activeElement;
  const selection = activeTextarea.value.substring(
    activeTextarea.selectionStart,
    activeTextarea.selectionEnd,
  );

  const outputElement = document.getElementById("output-element");
  const outputText = document.getElementById("output-text");
  outputElement.textContent = activeTextarea.id;
  outputText.textContent = selection;
}

const textarea1 = document.getElementById("ta-example-one");
const textarea2 = document.getElementById("ta-example-two");
textarea1.addEventListener("mouseup", onMouseUp, false);
textarea2.addEventListener("mouseup", onMouseUp, false);