SOURCE

function findNodeWithMostChildren(data, level = 0, index = 0) {
  if (!data.children || data.children.length === 0) {
    return { level, index };
  }

  let maxChildren = 0;
  let maxChildIndex = 0;

  for (let i = 0; i < data.children.length; i++) {
    const child = data.children[i];
    if (child.children && child.children.length > maxChildren) {
      maxChildren = child.children.length;
      maxChildIndex = i;
    }
  }

  if (maxChildren > 0) {
    return findNodeWithMostChildren(data.children[maxChildIndex], level + 1, maxChildIndex);
  } else {
    return { level, index };
  }
}

const demoData = {
  id: '1',
  name: '1',
  children: [
    { id: '1-1', name: '1-1' },
    { id: '1-1', name: '1-1' },
    {
      id: '1-2',
      name: '1-2',
      children: [
        { id: '1-2-1', name: '1-2-1' },
        { id: '1-2-2', name: '1-2-2' },
        { id: '1-2-3', name: '1-2-3' },
      ],
    },
    {
      id: '1-3',
      name: '1-3',
      children: [{ id: '1-3-1', name: '1-3-1' }],
    },
  ],
};

const result = findNodeWithMostChildren(demoData);
console.log(`Level: ${result.level}, Index: ${result.index}`);