Notate Depth in JS

depth.js

var obj = {"isbn": "123-456-222",  
 "author": 
    {
      "lastname": "Doe",
      "firstname": "Jane"
    },
"editor": 
    {
      "lastname": "Smith",
      "firstname": "Jane"
    },
  "title": "The Ultimate Database Study Guide",  
  "category": {
    "age": {
      "born": "1981",
      "month": {
        "name": "Jan"
      }
    }
  }
 };


const notate_depth = obj => {
  const nd_internal = (obj, depth) => {
     let keys = Object.keys(obj);
    return keys.map(k => {
      if ((typeof obj[k]) === 'object') {
        return nd_internal(obj[k], depth+1);
      } else {
        return [k, depth];
      }
    });
  };
  
  return nd_internal(obj, 0);
}