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components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNameBindings: ['active'],
  
  activeObserver: Ember.on('init', Ember.observer('active', function (){
  	Ember.run.scheduleOnce('afterRender', () => {
      const prevDivider = $(this.element.closest('.divider')).prev('.divider')
      const nextDivider = $(this.element.closest('.divider')).next('.divider')

      console.log(nextDivider)
        if (this.active) {
          prevDivider.addClass('before-active')
          nextDivider.addClass('after-active')
        } else {
          prevDivider.removeClass('before-active')
          nextDivider.removeClass('after-active')
        }
    })
  }))
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  activeFoo: false,
  activeBar: false,
  activeBiz: false,
  
  actions: {
    activeFoo () {
      this.setProperties({
      	activeFoo: true,
        activeBar: false,
				activeBiz: false
      })
  	},
                                       
    activeBar () {
			this.setProperties({
      	activeFoo: false,
        activeBar: true,
				activeBiz: false
      })
  	},
    
    activeBiz () {
			this.setProperties({
      	activeFoo: false,
        activeBar: false,
				activeBiz: true
      })
  	}
	}
});

styles.app.css

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

.active {
  color: red;
}

.before-active {
  background-color: pink;
}

.after-active {
  background-color: yellow;
}

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<button {{action (action 'activeFoo')}}>active foo</button>
<button {{action (action 'activeBar')}}>active Bar</button>
<button {{action (action 'activeBiz')}}>active Biz</button>

<div class='divider'>
{{my-component text='foo' active=activeFoo}}
</div>

<div class='divider'>
{{my-component text='bar' active=activeBar}}
</div>

<div class='divider'>
{{my-component text='biz' active=activeBiz}}
</div>



<br>
<br>

templates.components.my-component.hbs

{{text}} {{active}}

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"
  }
}