New Twiddle

components.page-loader.js

import Ember from 'ember';

export default Ember.Component.extend({
  increment: 1,
  index: 0,
  
  init() {
    this._super(...arguments);
    this.set('items', [0, 1, 2, 3, 4, 5, 6, 7]);
  },
  
  didInsertElement() {
    this._super(...arguments);
    
    const update = () => {
       const { index, items } = this;

      if (index >= items.length -1) {
        this.set('increment', -1);
      } else if (index <= 0) {
        this.set('increment', 1);
      }

      const newIndex = index + this.increment;
      this.set('index', newIndex);
      
      Ember.run.later(() => update(), 500);
    }
    
    Ember.run.later(() => update(), 500);
  }
});

controllers.application.js

import Ember from 'ember';

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

helpers.circle-style.js

import Ember from 'ember';

export function circleStyle([index, i] /*, hash*/) {
  return `transform: "translateX(${(index - i) * 40}px)"`;
}

export default Ember.Helper.helper(circleStyle);

styles.app.css

body {
  margin: 12px 16px;
  font-family: 'Helvetica Neue', Helvetica, Arial, sans-serif;
  font-size: 12pt;
}

.App {
  height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
}

.background {
  display: flex;
}

.circle {
  width: 40px;
  height: 40px;
  background-color: #ddd;
  position: relative;
  border-radius: 20px;
  margin-right: 4px;
  overflow: hidden;
}

.mover {
  position: absolute;
  top: 0;
  left: 0;
  width: 40px;
  height: 40px;
  border-radius: 20px;
  background-color: tomato;
  transition: transform .5s ease;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{page-loader}}
<br>
<br>

templates.components.page-loader.hbs

<div className="App">
  <div className="background">
    {{#each this.items as |i|}}
      <div className="circle">
        <div
          className="mover"
          style={{circle-style index i}}
        />
      </div>
    {{/each}}
  </div>
</div>

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"
  }
}