mut problem

helpers.add.js

import Ember from 'ember';

export function add(numbers) {
  return numbers.reduce((sum, n) => sum + n);
}

export default Ember.Helper.helper(add);

templates.application.hbs

{{#with (hash count=0) as |data|}}
  <button onclick={{action (mut data.count) (add data.count 1)}}>
    {{data.count}}
  </button>
{{/with}}

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": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}