Keeping a consistent parentId/id relationship with a flatten function

Suppose I have a nested object list like this:

[{
    "id": "a",
    "name": "Object a",
    "parentId": "root",
    "children": [{
        "id": "c",
        "name": "Object c"
    },{
        "id": "d",
        "name": "Object D",
        "parentId": "a"
    }]
}, {
    "id": "b",
    "name": "Object b",
    "parentId": "root"
}]

The object anatomy is simple: id, name, children (if any) and parentId. I’m using this flatten function that turns the nested object into a flat array:

 function flatten(array) {
  var result = [];
  array.forEach(function (a) {
      result.push(a);
      if (Array.isArray(a.children)) {
          result = result.concat(flatten(a.children));
      }
  });
  return result;
}

The thing is that the parentId value is not always persistent for every object, and therefore when the object gets flattened into an array, it’s possible to lose a parent & child object relationship.

I need the flatten method to rebuild the parentId value according to the object structure. And there’s only one catch, if the object is not a child, then it should have a parentId of root.

Help is very much appreciated

Answer

If I follow correctly, you just want to flatten your structure into an array that maintains the parentId relationship inherent in the original tree. If so, then I believe this will do:

const flatten = (xs, parentId = 'root') =>
  xs .flatMap (({children = [], id, ...rest}) => [
    {id, ...rest, parentId}, 
    ... flatten (children, id)
  ])

const input = [{id: "a", name: "Object a", parentId: "root", children: [{id: "c", name: "Object c"},{id: "d", name: "Object D", parentId: "a"}]}, {id: "b", name: "Object b", parentId: "root"}]

console .log (flatten (input))

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It’s a fairly simple recursion, using flatMap to combine the records and parameter destructuring (with default a parameter for children) to simplify our object handling.