deep node clone.js

const getStackTo = (node, find, stack = []) => {
  if (node === find) {
    return stack;
  }
  if (!node.children) {
    return;
  }
  if (node.children === find) {
    return [...stack, 'children'];
  }
  for (let i = 0; i < node.children.length; i++) {
    const child = node.children[i];
    const found = getStackTo(child, find, [...stack, 'children', i]);
    if (found) {
      return found;
    }
  }
};

const replaceAtStack = (node, [key, ...rest], replacement) => {
  return Array.isArray(node)
    ? [
        ...node.slice(0, key),
        rest.length === 0
          ? replacement
          : replaceAtStack(node[key], rest, replacement),
        ...node.slice(key + 1)
      ]
    : {
        ...node,
        [key]:
          rest.length === 0
            ? replacement
            : replaceAtStack(node[key], rest, replacement)
      };
};

const replaceNodeInAst = (ast, node, replacement) => {
  const stack = getStackTo(ast, node);
  if (!stack) {
    throw new Error('Node not found in ast!');
  }
  return replaceAtStack(ast, stack, replacement);
};

const a = {
  children: [
    {
      t: '2'
    },
    {
      y: '3',
      children: [
        'hi',
        {
          t: 4
        }
      ]
    }
  ],
  parent: 2
};
const q = a.children[1].children;

console.log(JSON.stringify(replaceNodeInAst(a, q, 'byeeee'), null, 2));
/**
 * logs
 * {
 *    "children": [
 *      {
 *        "t": "2"
 *      },
 *      {
 *        "y": "3",
 *        "children": "byeeee"
 *      }
 *    ],
 *    "parent": 2
 *  }
 **/