I tried the following code in JSBin, the first one could get the canvas object. However, the second one cannot.

JSBin: <http://jsbin.com/natavejasa/1/edit?html,js,output>

```
var canvas = $window.document.getElementById('myCanvas');

```

JSBin: <http://jsbin.com/yareb/1/edit?html,js,output>

```
var canvas = $document.getElementById('myCanvas');

```

So I'm wondering what's the difference between `$window.document` and `$document`.