New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  queryParams: ['foo'],
  foo: null,
  fooObj: Ember.computed('foo', {
    get() {
			return JSON.parse(this.foo);
    },
    set(k, val) {
      this.set('foo', JSON.stringify(val));
      return val;
    }
  }),
  init() {
    Ember.run.later(() => this.set('fooObj', { bar: 'start' }));
  },
  actions: {
    test() {
      this.set('fooObj', { bar: 'done' });
    }
  }
});

templates.application.hbs

<button onclick={{action 'test'}}>test</button>
{{fooObj.bar}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}