Computed from Array

controllers.application.js

import Ember from 'ember';

const { computed } = Ember;

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  model: Ember.Object.create({
    profile: Ember.Object.create({
      toolbox: [],
      activeToolbox: []
    })
  }),
  
  inTheBoxCount: computed('model.profile.toolbox.[]', function() {
    return this.get('model.profile.toolbox.length');
  }),
  
  actions: {
    addTool(name) {
      this.get('model.profile').get('toolbox').pushObject({
        name
      });
    },
    
    removeTool(tool) {
      this.get('model.profile').get('toolbox').removeObject(tool);
    },
    
    makeActive(tool) {
      this.get('model.profile').get('activeToolbox').pushObject(tool);
    }
  }
});

helpers.my-helper.js

import Ember from 'ember';

export function myHelper(params/*, hash*/) {
  let [tool, tools] = params;
  return tools.includes(tool);
}

export default Ember.Helper.helper(myHelper);

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
<button onclick={{action 'addTool' 'Hammer'}}>Add Hammer</button>

<p>Toolbox:</p>
<ul>
  {{#each model.profile.toolbox as |tool|}}
    <li><button onclick={{action 'removeTool' tool}}>Remove Hammer</button> - is in use {{my-helper tool model.profile.activeToolbox}} <button onclick={{action 'makeActive' tool}}>Make active</button></li>
  {{/each}}
</ul>
<p>Active tools length: {{model.profile.activeToolbox.length}}</p>
<p>Number for tools: {{inTheBoxCount}}</p>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.8.2",
    "ember-template-compiler": "2.8.2",
    "ember-testing": "2.8.2"
  },
  "addons": {
    "ember-data": "2.4.3"
  }
}