title: Array.push() and unique items
body: <p>I have a simple case of pushing unique values into array. It looks like this:</p>
  
  <pre><code>  this.items = [];
  
    add(item) {
      if(this.items.indexOf(item) &gt; -1) {
        this.items.push(item);
        console.log(this.items);
      }
    }
  </code></pre>
  
  <p>Seems pretty straight-forward, right? No, as it appears. It doesn't add any values. I am sure it's some kind of silly mistake on my side, but I can't seem to find it.</p>