Create nested data (JSON or Array)

Question

I have a parts list in the following structure:

level | description
    0 | sfg
    1 | sdf
    2 | asdisg
    3 | sidjhf
    4 | wefdsj
    4 | asdfg
    4 | sgfd
    4 | sdg
    3 | sfg
    3 | fdg
    4 | sdfg
    4 | sdsfd
    4 | sdf
    4 | sdf
    2 | adf
    3 | sdfg
    4 | sdg
    4 | sdf
    4 | sdf
    4 | sdf
    1 | shfg
    2 | ijsd
    3 | ijsd
    4 | sdf
    4 | sdf

What I need is to convert this part list into a hierarchy folder structure like this (prefered in nested JSON):

I want to use the following structure in TypeScript:

export interface Foo {
  level: number;
  description: string;
  children?: Foo[];
}

What I need is a nested JSON object or something like a nested array in which the array contains children with another array.

Unfortunately I have only pseudocode to show (I know, this will never work):

  recursiveWalker(List, lastLevel, parent) {
    for (let i = 0; i < List.length; ++i) {
      const node = {
        description: List.description,
        level: List.level
      };
      if (List.level === lastLevel + 1) {
        node.children = this.recursiveWalker(List, lastLevel + 2, node);
      }
    }
    return node;
  }

My question is now: Is there a simple solution to create such nested data? Do I have to use recursive functions, or are there any algorithms which I can implement?

The goal is to create a material tree in Angular (https://material.angular.io/components/tree/overview) with this data.

I really appreciate any kind of help!

UPDATE: The following structure will be needed:

  const bar: Foo = {
    description: 'this is level 0 (root)',
    level: 0,
    children: [{
      description: 'this is level 1',
      level: 1,
      children: [
        {
          description: 'this is level 2',
          level: 2
        },
        {
          description: 'this is level 2',
          level: 2
        },
        {
          description: 'this is level 2',
          level: 2,
          children: [
            {
              description: 'this is level 3',
              level: 3
            },
            {
              description: 'this is level 3',
              level: 3
            },
            {
              description: 'this is level 3',
              level: 3,
              children: [] /* AND SO ON .... */
            }
          ]
        }
      ]
    }]
  };
});

how do you now which next level goes under which parent? please add a wanted data structure which matches the input data. — Nina Scholz
– Nina Scholz, Commented Nov 21, 2019 at 8:46
@NinaScholz in the same order as it comes. I will update my post. — Toberto
– Toberto, Commented Nov 21, 2019 at 8:50

Nina Scholz · Accepted Answer · 2019-11-21 09:41:52Z

You could take a lazy approach by taking an array levels as helper for keeping track of the latest nodes of the actual level.

var data = [{ level: 0, description: '0' }, { level: 1, description: '0-0' }, { level: 2, description: '0-0-0' }, { level: 3, description: '0-0-0-0' }, { level: 4, description: '0-0-0-0-0' }, { level: 4, description: '0-0-0-0-1' }, { level: 4, description: '0-0-0-0-2' }, { level: 4, description: '0-0-0-0-3' }, { level: 3, description: '0-0-0-1' }, { level: 3, description: '0-0-0-2' }, { level: 4, description: '0-0-0-2-0' }, { level: 4, description: '0-0-0-2-1' }, { level: 4, description: '0-0-0-2-2' }, { level: 4, description: '0-0-0-2-3' }, { level: 2, description: '0-0-1' }, { level: 3, description: '0-0-1-0' }, { level: 4, description: '0-0-1-0-0' }, { level: 4, description: '0-0-1-0-1' }, { level: 4, description: '0-0-1-0-2' }, { level: 4, description: '0-0-1-0-3' }, { level: 1, description: '0-1' }, { level: 2, description: '0-1-0' }, { level: 3, description: '0-1-0-0' }, { level: 4, description: '0-1-0-0-0' }, { level: 4, description: '0-1-0-0-1' }],
    tree = [],
    levels = [tree];
   
data.forEach(({ level, description }) =>
    levels[level].push({ level, description, children: levels[level + 1] = [] })
);

console.log(tree);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Oh thank you very much @NinaScholz! This is such a good approach. It works fine. I marked the answer as accepted. You saved my day :)

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