rot13 the current selection in your browser

rot13-inline.js

(function () {
  function rot13(str) {
    return str.replace(/([A-Ma-m])|([N-Zn-z])/g, function (m, p1, p2) {
      return String.fromCharCode(m.charCodeAt(0) + (p1 ? 13 : -13));
    });
  }
  if (document.getSelection().rangeCount == 0) {
    return;
  }
  const range = window.getSelection().getRangeAt(0);
  const iter = document.createNodeIterator(
    range.commonAncestorContainer,
    NodeFilter.SHOW_TEXT
  );
  let el = iter.nextNode();
  let more = true;
  while (more) {
    if (
      el.compareDocumentPosition(range.startContainer) &
        Node.DOCUMENT_POSITION_FOLLOWING ||
      el.textContent.trim() === ""
    ) {
      el = iter.nextNode();
      more = !!el;
      continue;
    }
    if (
      el.compareDocumentPosition(range.endContainer) &
      Node.DOCUMENT_POSITION_PRECEDING
    ) {
      break;
    }
    let replacementText;
    if (el === range.startContainer && el === range.endContainer) {
      replacementText =
        el.textContent.substring(0, range.startOffset) +
        rot13(el.textContent.substring(range.startOffset, range.endOffset))
        + el.textContent.substring(range.endOffset);
    } else if (el === range.startContainer) {
      replacementText =
        el.textContent.substring(0, range.startOffset) +
        rot13(el.textContent.substring(range.startOffset));
    } else if (el === range.endContainer) {
      replacementText =
        rot13(el.textContent.substring(0, range.endOffset)) +
        el.textContent.substring(range.endOffset);
    } else {
      replacementText = rot13(el.textContent);
    }
    let elToReplace = el;
    el = iter.nextNode();
    more = !!el;
    elToReplace.replaceWith(document.createTextNode(replacementText));
  }
  window.getSelection().removeAllRanges();
})();