walk.ts

function $walk(
  start: LexicalNode | null,
  isBackward: boolean = false
): LexicalNode | null {
  let node: LexicalNode | null = start;
  if ($isElementNode(node) && !node.isEmpty()) {
    return isBackward ? node.getLastChild() : node.getFirstChild();
  }

  let sibling: LexicalNode | null = null;
  while (sibling === null && node !== null) {
    sibling = isBackward ? node.getPreviousSibling() : node.getNextSibling();
    node = sibling || node.getParent();
  }

  return node;
}

editor.getEditorState().read(() => {
  let node: LexicalNode = startNode;
  
  // while or do-while depending on whether start node needs to be processed too
  do {
    // break whenever found proper node
    console.log(node);
  } while (node = $walk(node))
});