Unable to update textarea value with formatted text

<textarea name="sum" id="sum"></textarea>

<script>
  const sum = document.getElementById('sum');

  sum.addEventListener('paste', () =>
    setTimeout(
      () => (sum.value = sum.value.replace(/[\r\n](?![\r\n])/g, '')),
      0
    )
  );
</script>

https://developer.mozilla.org/en-US/docs/Web/API/Element/paste_event

<textarea> is not supported .innerHTML so you need to set values through .value and you can use oninput event.

This event occurs when the value of an <input> or <textarea> element is changed.

Helpful links:
https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers/oninput https://www.w3schools.com/jsref/event_oninput.asp

function frmt(id){    
    const tar = document.getElementById(id);
    const reg = /[\r\n](?![\r\n])/g;
    const str = tar.value;
    tar.value = str.replace(reg, "")
}

textarea{
  width: 95%;
  height: 160px;
}

<textarea name="sum" id="sum" oninput="frmt('sum')"></textarea>