Class RangeElement

const rangeElement = DocumentApp.getActiveDocument().getSelection().getRangeElements()[0];

// Log debug information about a range element.
Logger.log(`Element type: ${rangeElement.getElement().getType()}`);
if (rangeElement.isPartial()) {
  Logger.log(`The character range begins at ${rangeElement.getStartOffset()}`);
  Logger.log(
      `The character range ends at ${rangeElement.getEndOffsetInclusive()}`,
  );
} else {
  Logger.log('The entire range element is included.');
}