Reverse Nested Tree in Javascript

Question

i have the following array of dictionaries with the one below as an example of a single dictionary

{
        "id": 80,
        "name": "Low Level Topic",
        "questions": [
            {
                "id": 23,
                "shortName": null,
                "name": "Some question"
            },
            {
                "id": 24,
                "shortName": null,
                "name": "Some question 2"
            }
        ],
        "parentTopic": {
            "id": 10,
            "name": "Higher Level Topic",
            "parentTopic": {
                 "id": 1,
                 "name": "Top Level Topic",
                 "parentTopic": null
                }
            }
        }
    }

and i would like to reverse the tree to end up with the following

{
    "id": 1,
    "name": "Top Level Topic",
    "childTopic": {
        "id": 10,
        "name": "Higher Level Topic",
        "childTopic": {
            "id": 80,
            "name": "Low Level Topic",
            "questions": [
                {
                    "id": 23,
                    "shortName": null,
                    "name": "Some question"
                 },
                 {
                     "id": 24,
                     "shortName": null,
                     "name": "Some question 2"
                 }
        ],
        }
    } 
}

i have been trying to find an easy way to regenerate/restructure the data, but was not successful. I would appreciate if anyone could help

Asraf · Accepted Answer · 2022-11-08 15:04:59Z

1

You can use simple recursion and it won't mutate the original obj

let obj = {"id":80,"name":"Low Level Topic","questions":[{"id":23,"shortName":null,"name":"Some question"},{"id":24,"shortName":null,"name":"Some question 2"}],"parentTopic":{"id":10,"name":"Higher Level Topic","parentTopic":{"id":1,"name":"Top Level Topic","parentTopic":null}}};

const buildReverseObj = (originalObj, currObj) => {
  const copyObj = { // make copy of original obj
    ...originalObj,
  };
  const parentTopic = copyObj.parentTopic; // get the parentTopic to process in next step
  delete copyObj.parentTopic; // parentTopic should be removed 

  const childTopic = { // make a child topic in every step it consists of older contents and newer contents
    ...copyObj,
    ...currObj
  };
  if (!parentTopic) return childTopic; // when parentTopic = null we are done

  return buildReverseObj(parentTopic, { childTopic });
};
console.log(buildReverseObj(obj, {}));

edited Nov 8, 2022 at 15:04

answered Nov 8, 2022 at 8:39

Asraf

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1 Comment

JimminyCricket Over a year ago

Thanks Asraf, this worked nicely :)

k.tten · Accepted Answer · 2022-11-07 23:12:37Z

This problem is essentially reversing a linked list, and changing its structure a little.

let data = {"id":80,"name":"Low Level Topic","questions":[{"id":23,"shortName":null,"name":"Some question"},{"id":24,"shortName":null,"name":"Some question 2"}],"parentTopic":{"id":10,"name":"Higher Level Topic","parentTopic":{"id":1,"name":"Top Level Topic","parentTopic":null}}};

// WARNING: this code MUTATES the object

// previous topic, current topic, new tail as 'tail'
let prev = null, curr = data, tail = data.parentTopic;

while (curr) {
    next = curr.parentTopic;
    delete curr.parentTopic; // rename parentTopic
    curr.childTopic = prev;  // to childTopic
    prev = curr;
    curr = next;
}

delete tail.childTopic.childTopic; // delete since it's null

data = prev; // assign new head to data

console.log(data);

.as-console-wrapper { max-height: 100% !important }

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