TrackedSet Helper

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';
import { inject as service } from '@ember/service';
import { TrackedSet } from 'tracked-built-ins';


export default class ApplicationController extends Controller {
  @service
  myService;
  
  myThings = new TrackedSet(new Set(['Thing 1', 'Thing 2']));
  
  get cart() {
    return this.myService.getInstanceOfCart();
  }
  
  @action
  newThing(evt) {
    this.myThings.add(evt.target.newThing.value);
  }
}

helpers.my-helper\.js

import Helper from '@ember/component/helper';
import { inject as service } from '@ember/service';


export default class MyHelper extends Helper {
  @service
  myService;
  
  get cart() {
    return this.myService.getInstanceOfCart();
  }
  
  compute([thing]) {
    return this.cart.isThingInCart(thing);  
  }
}

services.my-service\.js

import Service from '@ember/service';
import { TrackedSet } from 'tracked-built-ins';
import { action } from '@ember/object';

class Cart {
  _mySet = new TrackedSet();
  
  @action
  isThingInCart(thing) {
    return this._mySet.has(thing.slice(2));
  }
  
  @action
  addThingToCart(thing) {
    this._mySet.add(thing.slice(2));
  }

  @action
  removeThingFromCart(thing) {
    this._mySet.delete(thing.slice(2));
  }
}

export default class MyService extends Service {
  _cart = new Cart();

  @action
  getInstanceOfCart() {
    return this._cart;
  }
}

templates.application\.hbs

<form {{on "submit" this.newThing}} action="javascript:void()">
  <label for="new-thing">New Thing</label>
  <input type="text" id="new-thing" name="newThing">
  <button>Add</button>
</form>

<ul>
  {{#each this.myThings as |thing|}}
    <li>
      {{thing}}
      {{#if (my-helper thing)}}
        <button type="button" {{on "click" (fn this.cart.removeThingFromCart thing)}}>Remove from my class</button>
      {{else}}
        <button type="button" {{on "click" (fn this.cart.addThingToCart thing)}}>Add to my class</button>
      {{/if}}
    </li>
  {{/each}}
</ul>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "tracked-built-ins": "1.0.2"
  }
}