Javascript in ionic template can't working

I have no idea why the script tag can't working, here is the reproduce step: ionic cli 1.3.10

1.ionic start mytab

2.add following to templates/chat-detail.htm in ion-content

<p id="demo"></p>

<script>
var txt="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
document.getElementById("demo").innerHTML = txt.length;
</script>

3.ionic serve --lab

4.navigate to chat detail page,the script couldn't work

why is that happened?