New Twiddle

components.child-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  actions: {
    clearPlaceholder() {
      this.set('placeholder', '');
    }
  }
});

components.parent-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  actions: {
    addPlaceholder() {
      this.set('placeholder', 'Hi Claudia');
    }
  }
});

controllers.application.js

import Ember from 'ember';

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

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{parent-component}}
<br>
<br>

templates.components.child-component.hbs

{{placeholder}}<br />
<button {{action "clearPlaceholder"}}>Clear</button>

templates.components.parent-component.hbs

<button {{action "addPlaceholder"}}>Reply</button>
{{child-component placeholder=placeholder}}

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"
  }
}