define-property-observer

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

import EmberObject, {
  computed,
    defineProperty,
  observer
} from '@ember/object';

let Person = EmberObject.extend({
  // these will be supplied by `create`
  firstName: null,
  lastName: null,

  fullName: computed('firstName', 'lastName', function() {
    return `${this.firstName} ${this.lastName}`;
  }),

  fullNameChanged2: observer('fullName', function() {
    console.log(`fullNameChanged2 fired: ${this.fullName}`);
  }),

  init() {
  	this._super(...arguments);
    defineProperty(this, 'fullNameChanged', observer('fullName', function() {
	    // deal with the change
	    console.log(`fullName changed to: ${this.fullName}`);
  	}));
	}
});

let person = Person.create({
  firstName: 'Yehuda',
  lastName: 'Katz'
});

// observer won't fire until `fullName` is consumed first
person.get('fullName'); // "Yehuda Katz"
person.set('firstName', 'Brohuda'); // fullName changed to: Brohuda Katz

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

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"
  }
}