Francesca_M on "post_id to javascript var"

Hi,
I'm looking for some help:
I need, when the user click on 'read more', to save the post id in a javascript var.

If i use:

<div id="post-<?php the_ID(); ?>">
<script type='text/javascript'>
	<?php
	$num_post="post-".get_the_ID();
	echo "var selected_post='$num_post";
	?>
	</script>

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the code write the right var for every post, but if I try to use it, I always have the id of the last post of the page. How can I save only the necessary var?