I have dynamically generated `div` that receives two generated classes applied to it. For example, I have :

```
<div class="container">...</div> 

```

Then after I mess with it using some Javascript, it becomes:

```
<div class="container post post-hello-world">...</div>

```

For this example how can I grab the third class (`post-hello-world`) and store it somewhere to be used for something else in my Javascript code using pure Javascript?