Emberjs Getting array property returns undefined

I want to define a reactive array property in a component so that every time the array is updated, it auto-updates necessary HTML content.

I thought this was going to be straightforward, but it didn't work as I expected. Every time I retrieve the property via this.get('array'), it returns undefined.

// components/test-component.js
export default Ember.Component.extend({
  _array: [],

  array: Ember.computed('_array',{
    get(key) { return this.get('_array'); },
    set(key,value) { this.set('_array', value); }
  }),

  isEmpty: Ember.computed('array', function() {
    // Here, this.get('array') returns undefined. Why?
    return this.get('array').length;
  }),

  actions: {
    addNew() {
      this.get('array').push(Date.now());
    }
  },

  init() {
    this._super(...arguments);

    this.set('array', [1,2,3]);
  },
});

I also noticed that in the init method, if I retrieve the array property right after setting it, it also returns undefined. Why is this happening?

Here is a twiddle. It is supposed to iterate the array, and show a list of all items, but it is currently crashing because it returns undefined.