construct PHP multidimensional array from Laravel object

Question

This is an example of a data list from Laravel DB:

Illuminate\Support\Collection Object
(
[items:protected] => Array
    (
        [0] => stdClass Object
            (
                [category] => Protections menstruelles
                [sub_category] => Sous-vêtements lavables
                [model] => Culotte menstruelle
            )

        [1] => stdClass Object
            (
                [category] => T-shirt, Top & Polo
                [sub_category] => Débardeur, Caraco
                [model] => Bustier
            )

        [2] => stdClass Object
            (
                [category] => Protections menstruelles
                [sub_category] => Sous-vêtements lavables
                [model] => Tanga menstruel
            )

        [3] => stdClass Object
            (
                [category] => Protections menstruelles
                [sub_category] => Sous-vêtements lavables
                [model] => Body menstruel
            )

        [4] => stdClass Object
            (
                [category] => Protections menstruelles
                [sub_category] => Maillot de bain menstruel
                [model] => 1 pièce
            )

        [5] => stdClass Object
            (
                [category] => Homewear
                [sub_category] => Chaussettes
                [model] => Socquettes
            )

        [6] => stdClass Object
            (
                [category] => Plage
                [sub_category] => Bas de maillot
                [model] => Short de bain
            )

        [7] => stdClass Object
            (
                [category] => Lingerie & Sous-vêtement
                [sub_category] => Soutien-gorge
                [model] => Triangle
            )

        [8] => stdClass Object
            (
                [category] => Lingerie & Sous-vêtement
                [sub_category] => Culotte, tanga & string
                [model] => Tanga
            )

        [9] => stdClass Object
            (
                [category] => Lingerie & Sous-vêtement
                [sub_category] => Culotte, tanga & string
                [model] => Culotte
            )

    )
)

And I would like to create a JSON object which could be like that:

        var subjectObject = {
          "Protections menstruelles": {
            "Maillot de bain menstruel": ["1 pièce"],
            "Sous-vêtements lavables": ["Body menstruel"]
          },
          "Lingerie & Sous-vêtement": {
            "Soutien-gorge": ["Triangle"],
            "Culotte, tanga & string": ["Tanga", "Culotte"]
          }
          ...
        }

The idea is to ask some questions in select and with JavaScript conditional the options according previous select. I want to use this tutorial for that: https://www.w3schools.com/howto/howto_js_cascading_dropdown.asp I need a help to construct the json input.

It's working with this laravel DB request :

->select('category','sub_category','model')
->whereNotNull('category')
->whereNotNull('sub_category')
->whereNotNull('model')
->distinct()
->get()
->groupBy('category')
->map(function($item){
    return collect($item)
        ->groupBy('sub_category')
        ->map(function($item){
            return collect($item)->pluck('model');
        })->toArray();
      })->toJson();

But i Can't use it in javascript :

var subjectObject = "{{ $collection }}";

It's not a json like in the example :

var subjectObject = {
 "Front-end": {
  "HTML": ["Links", "Images", "Tables", "Lists"],
  "CSS": ["Borders", "Margins", "Backgrounds", "Float"],
  "JavaScript": ["Variables", "Operators", "Functions", "Conditions"]    
 },
 "Back-end": {
   "PHP": ["Variables", "Strings", "Arrays"],
   "SQL": ["SELECT", "UPDATE", "DELETE"]
 }
}