仅在页面加载时打印Iframe内容

Question

我正在尝试在加载时打印Iframed内容。但是我的代码不起作用。任何帮助将不胜感激。

<script>
window.onload = function() {
var newWin = document.getElementById('print').contentWindow;
newWin.document.write(body);
newWin.document.close(); //important!
newWin.focus(); //IE fix
newWin.print();
}
</script>

<iframe id="print" src="http://www.davearbogast.com/wp-content/uploads/2015/08/MAINTENANCESPECIALCoupon.jpg" width="300" height="600" frameborder="0"></iframe>