How to access just the changed data of an 'input' event regardless of text deletion or insertion like pasted or typed or restored text for the latter?

// the node reference based storage of most recent element values.
const mostRecentValueStorage = new WeakMap;

function getDataChangeFromDeleteOrPaste({ currentTarget, data }) {
  const recentValue = mostRecentValueStorage.get(currentTarget);
  const { value: currentValue, selectionEnd } = currentTarget;

  let deletionStart = selectionEnd;
  let leadingValue = currentValue.slice(0, deletionStart);

  while ((leadingValue !== '') && !recentValue.startsWith(leadingValue)) {
    leadingValue = leadingValue.slice(0, --deletionStart);
  }
  const deletionLength =
    (selectionEnd - deletionStart) + (recentValue.length - currentValue.length);

  const deleted = recentValue.slice(deletionStart, (deletionStart + deletionLength));

  const insertLength = currentValue.length + deleted.length - recentValue.length;
  const insertStart = selectionEnd - insertLength;

  const inserted = (insertStart < selectionEnd)
    && currentValue.slice(insertStart, selectionEnd)
    || null;

  return {
    currentValue,
    recentValue,
    deleted: (deleted === '') ? null : deleted,
    deletionStart: (deleted === '') ? null : deletionStart,
    deletionLength: (deleted === '') ? null : deletionLength,
    inserted,
    insertStart: (inserted === null) ? null : insertStart,
    insertLength: (inserted === null) ? null : insertLength,
  };
}
function getDataChangeFromInsertText({ currentTarget, data }) {
  const recentValue = mostRecentValueStorage.get(currentTarget);
  const { value: currentValue, selectionEnd } = currentTarget;

  const insertLength = data.length;

  const deletionStart = (selectionEnd - insertLength);
  const deletionLength = (recentValue.length - currentValue.length + insertLength);

  const deleted = recentValue.slice(deletionStart, (deletionStart + deletionLength));

  return {
    currentValue,
    recentValue,
    deleted: (deleted === '') ? null : deleted,
    deletionStart: (deleted === '') ? null : deletionStart,
    deletionLength: (deleted === '') ? null : deletionLength,
    inserted: data,
    insertStart: (selectionEnd - insertLength),
    insertLength,
  };
}

function handleCustomInputDataChange(evt) {
  const { currentTarget } = evt;

  if (currentTarget.value !== mostRecentValueStorage.get(currentTarget)) {
    const { data } = evt;

    const dataChange = (
      (typeof data === 'string') && getDataChangeFromInsertText(evt)
    ) || (
      (data === null) && getDataChangeFromDeleteOrPaste(evt)
    ) || null;

    // put the most recent element value into an object based storage.
    mostRecentValueStorage.set(currentTarget, currentTarget.value);

    currentTarget
      .dispatchEvent(
        new CustomEvent('input:datachange', {
          bubbles: true,
          detail: {
            dataChange,
            inputEvent: evt,
          },
        })        
      );
  }
}

function enableCustomInputDataChangeHandling(elmNode) {
  // put the initial element value into an object based storage.
  mostRecentValueStorage.set(elmNode, elmNode.defaultValue);

  elmNode
    .addEventListener('input', handleCustomInputDataChange);
}
function initializeCustomInputDataChangeHandling() {
  document
    // for every element which features a
    // `data-handle-input-data-change` attribute ...
    .querySelectorAll('[data-handle-input-data-change]')

    // ... enable the handling of a custom
    // 'input:datachange' event.
    .forEach(enableCustomInputDataChangeHandling);
}

function main() {
  initializeCustomInputDataChangeHandling();

  document
    // subscribe to the custom 'input:datachange' event wherever it is needed. 
    .addEventListener(
      'input:datachange',
      ({ target, detail: { dataChange } }) => console.log({ /*target, */dataChange })
    );
}
main();

body { margin: 0; }
.as-console-wrapper { left: auto!important; width: 76%; min-height: 100%!important; }

<textarea
  data-handle-input-data-change
  cols="16"
  rows="12"
>The quick brown fox jumps over the lazy dog ... edit text in whichever way.</textarea>