classnames

components.my-component.js

import Ember from 'ember';
import computed, { equal } from 'ember-computed';

export default Ember.Component.extend({
  tagName: '',
  didRender() {
    // show classes in DOM for debug purposes
    $('body > .my-debug').remove();
    $('body').append(`<div class="my-debug">${$('button').attr('class')}</div>`);
  }
});

controllers.application.js

import Ember from 'ember';

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

helpers.eq.js

import Ember from 'ember';

export function myHelper([a, b]/*, hash*/) {
  return a === b;
}

export default Ember.Helper.helper(myHelper);

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<div>
  withIcon: {{input type="checkbox" checked=withIcon}}
</div>
<div>
  buttonType: {{input value=buttonType}}
</div>
<div>
  onlyIcon: {{input type="checkbox" checked=onlyIcon}}
</div>
<div>
  fullWidth: {{input type="checkbox" checked=fullWidth}}
</div>
<div>
  iconType: {{input value=iconType}}
</div>
{{#my-component withIcon=withIcon buttonType=buttonType onlyIcon=onlyIcon fullWidth=fullWidth iconType=iconType}}Hello world{{/my-component}}
<br>
<br>

templates.components.my-component.hbs

<button class="
    {{if buttonType (concat 'Button--' buttonType)}}
    {{if onlyIcon 'Button--icon'}}
    {{if withIcon 'Button--withIcon'}}
    {{if iconType (concat 'Icon--' iconType)}}
    {{if (eq buttonType 'red') 'Button--outline'}}
    {{if fullWidth 'Button--fullWidth'}}
  ">
  {{yield}}
</button>

twiddle.json

{
  "version": "0.8.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.5.1",
    "ember-data": "2.5.2",
    "ember-template-compiler": "2.5.1"
  }
}