Javascript to manipulate document objects

I'm trying to manipulate some elements on my programD pages, such as images by usin the Javascript tag

but doesn't work

the code is something like this

<template>Showing the image
<javascript>
var capa = document.getElementById("muestra")
capa.style.visibility = "visible"

</javascript>
</template>



but it gives me this error on the activity log

[2007-10-04 19:36:32,687] WARN: Got exception:
ReferenceError: "document" is not defined.

it this could work? am I doing any thing wrong?

I also need to open a popup window like window.open("whatever")

Thanks