Get first level, second level and third level child in Node JS

Question

I have a dropdown in UI side and list of the objects. In this list there are lots of child objects and I need to filter all of those children. Please check below object. I am trying to do by Node JS.

 [
    {
        name : "This is horse",
        id:1
        parentId : null
    },
    {
        name : "horse child",
        id:2
        parentId : 1
    },
    {
        name : "horse child child",
        id:3
        parentId : 2
    },
    {
        name : "QQQQQ",
        id:4
        parentId : 1
    },
    {
        name : "WWWWWWWWWW",
        id:5
        parentId : 3
    },
    {
        name : "WWWWWWWWWW",
        id:6
        parentId : null
    }

]

If I select Level 0 then I get id: 1,6
If I select Level 1 then I get id: 1,2,4,6
If I select Level 2 then I get id: 1,2,3,4,5,6

_"I am trying to do by Node JS but its not working."_ - please add this effort as a text-based [mcve] — evolutionxbox, Aug 24 '21 at 12:41
What does "level" mean? Apparently it's cumulative? Because if I asked for all of the level 2 items, I would expect to get only those children that had grand-parents, not their parents and the grand-parents themselves. — Heretic Monkey, Aug 24 '21 at 12:50
@HereticMonkey, Yes you are correct but we need all of those. That why i am stuck also — Pritesh Mahajan, Aug 24 '21 at 12:52
You might find inspiration from [Get all children from parent child JSON data](https://stackoverflow.com/q/39976089/215552) and [Recurvisely find children of children and remove](https://stackoverflow.com/q/56133354/215552) (just don't remove anything) — Heretic Monkey, Aug 24 '21 at 13:05
Just convert the object to a nested tree structure. Don't try to do repeated level queries on this structure. — trincot, Aug 24 '21 at 13:11
You need just the `id` values or the corresponding objects? Do you need them in a specific order? — trincot, Aug 24 '21 at 13:22

score 0 · Answer 1 · answered Aug 24 '21 at 14:29

You should do some preprocessing on the data, so each object will get a level property. Once you have that, it is piece of cake to filter with:

result = data.filter(o => o.level <= givenLevel);

Use a tree building algorithm to populate your data with level. This code will also add a children property to each node. Maybe it can be useful:

function addLevels(data) {
    let map = new Map(data.map(o => [o.id, Object.assign(o, {children: []})])).set(null, {children: []});
    for (let o of data) map.get(o.parentId).children.push(o.id);
    (function dfs(id=null, level=-1) {
        const node = map.get(id);
        node.level = level;
        for (let child of node.children) dfs(child, level + 1);
    })();
}

// Example data
const data = [{ name : "This is horse", id:1, parentId : null }, { name : "horse child", id:2, parentId : 1 }, { name : "horse child child", id:3, parentId : 2 }, { name : "QQQQQ", id:4, parentId : 1 }, { name : "WWWWWWWWWW", id:5, parentId : 3 }, { name : "WWWWWWWWWW", id:6, parentId : null } ];

addLevels(data);

console.log(data);

You would only do this once (or every time the data changes), not every time you want to filter the data by level.

Get first level, second level and third level child in Node JS

1 Answers1