stackoverflow-55261082

controllers.application.js

import Controller from '@ember/controller';
import { computed } from '@ember/object';
import { inject as injectService } from '@ember/service';

export default Controller.extend({
  currentSession: injectService(),

  bar: computed('currentSession.foo', function() {
    return this.currentSession.foo;
  }),
  
  actions: {
    change() {
      this.currentSession.set('foo', 'changed');
    }
  }
});

services.current-session.js

import Service from '@ember/service';

export default Service.extend({
  foo: 'foo bar baz',
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
Property of service: {{currentSession.foo}}
<br>
Alias in controller: {{currentSession.foo}}
<br>
<button onclick={{action "change"}}>mutate property</button>

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"
  }
}