Build tree array from flat array in JavaScript

We have a complex json file that we have to handle with JavaScript to make it hierarchical, in order to later build a tree.

Every entry of the JSON array has −

• id − a unique id,

• parentId − the id of the parent node (which is 0 if the node is a root of the tree)

• level − the level of depth in the tree

The JSON data is already "ordered", means that an entry will have above itself a parent node or brother node, and under itself a child node or a brother node.

The input array is −

const arr = [
   {
      "id": "12",
      "parentId": "0",
      "text": "Man",
      "level": "1",
      "children": null
   },
   {
      "id": "6",
      "parentId": "12",
      "text": "Boy",
      "level": "2",
      "children": null
   },
   {
      "id": "7",
      "parentId": "12",
      "text": "Other",
      "level": "2",
      "children": null
   },
   {
      "id": "9",
      "parentId": "0",
      "text": "Woman",
      "level": "1",
      "children": null
   },
   {
      "id": "11",
      "parentId": "9",
      "text": "Girl",
      "level": "2",
      "children": null
   }
];

And the expected output is −

const output = [
   {
      "id": "12",
      "parentId": "0",
      "text": "Man",
      "level": "1",
      "children": [
         {
            "id": "6",
            "parentId": "12",
            "text": "Boy",
            "level": "2",
            "children": []
         },
         {
            "id": "7",
            "parentId": "12",
            "text": "Other",
            "level": "2",
            "children": []
         }
      ]
   },
   {
      "id": "9",
      "parentId": "0",
      "text": "Woman",
      "level": "1",
      "children": [
         {
            "id": "11",
            "parentId": "9",
            "text": "Girl",
            "level": "2",
            "children": []
         }
      ]
   }
];

Example

The code for this will be −

const arr = [
   {
      "id": "12",
      "parentId": "0",
      "text": "Man",
      "level": "1",
      "children": null
   },
   {
      "id": "6",
      "parentId": "12",
      "text": "Boy",
      "level": "2",
      "children": null
   },
   {
      "id": "7",
      "parentId": "12",
      "text": "Other",
      "level": "2",
      "children": null
   },
   {
      "id": "9",
      "parentId": "0",
      "text": "Woman",
      "level": "1",
      "children": null
   },
   {
      "id": "11",
      "parentId": "9",
      "text": "Girl",
      "level": "2",
      "children": null
   }
];
const listToTree = (arr = []) => {
   let map = {}, node, res = [], i;
   for (i = 0; i < arr.length; i += 1) {
      map[arr[i].id] = i;
      arr[i].children = [];
   };
   for (i = 0; i < arr.length; i += 1) {
      node = arr[i];
      if (node.parentId !== "0") {
         arr[map[node.parentId]].children.push(node);
      }
      else {
         res.push(node);
      };
   };
   return res;
};
console.log(JSON.stringify(listToTree(arr), undefined, 4));

Output

And the output in the console will be −

[
   {
      "id": "12",
      "parentId": "0",
      "text": "Man",
      "level": "1",
      "children": [
         {
            "id": "6",
            "parentId": "12",
            "text": "Boy",
            "level": "2",
            "children": []
         },
         {
            "id": "7",
            "parentId": "12",
            "text": "Other",
            "level": "2",
            "children": []
         }
      ]
   },
   {
      "id": "9",
      "parentId": "0",
      "text": "Woman",
      "level": "1",
      "children": [
         {
            "id": "11",
            "parentId": "9",
            "text": "Girl",
            "level": "2",
            "children": []
         }
      ]
   }
]