SOURCE

const data= [
  {
    label: 'Level one 1',
    children: [
      {
        label: 'Level two 1-1',
        children: [
          {
            label: 'Level three 1-1-1',
          },
        ],
      },
    ],
  },
  {
    label: 'Level one 2',
    children: [
      {
        label: 'Level two 2-1',
        children: [
          {
            label: 'Level three 2-1-1',
          },
        ],
      },
     
    ],
  }
]
function preOrder(list, result=[], labels=[]) {
  return list.reduce((res,v) => {
    labels.push(v.label);
    if(v.children && v.children.length) preOrder(v.children, res, labels);
    else res.push(labels.join(' > '))
    labels.pop();
    return res;
  }, result)
}

console.log(preOrder(data))