javascript iframe listener

1var iframe = document.getElementById('myIFrame');
2iframe.contentWindow.body.addEventListener('mouseup', Handler);
3
4function Handler() {
5    alert('works');
6}

1window.addEventListener('load', (event) => {
2
3var ifrm = = document.getElementById('myIFrame');
4  
5ifrm.contentWindow.document.addEventListener('mouseup', Handler);
6
7function Handler() {
8    alert('works');
9}
10
11});