Check dynamic classNames

components.my-component.js

import Ember from 'ember';

export default Ember.Component.extend({
  classNames: ['hello']
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  foo: true,
  
  actions: {
   	changeFoo() {
     	this.set('foo', !this.get('foo')); 
      console.log(this.get('foo'));
    }
  }
});

styles.app.css

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

.hello {
  border: 4px solid black; 
}

.foo {
  background-color: red; 
}

.bar {
  background-color: green;
}

templates.application.hbs

<h1>Dynamic Classnames</h1>
<br>
<br>
{{my-component
	class=(concat 'nothing ' (if foo 'foo' 'bar'))
}}
<br>
<br>
<button {{action 'changeFoo'}}>Change</button>

templates.components.my-component.hbs

<br />
<br />
Hello
<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"
  }
}