By default this information isn't (reliably, stably) exposed via Javascript. You're going to need to expose it yourself via a custom block added to the layout. The easiest way to do this will be
Adding the block via your theme's local.xml file
Adding a template to your theme for the above block
To add the block to the layout via your local.xml file, something like this should suffice
<default>
<reference name="root">
<block name="my_custom_js_block">
<action method="setTemplate">
<template>my_custom_js_block/extra-js.phtml</template>
</action>
</block>
</reference>
</default>
Then add the following folder and file to your theme
app/design/frontend/default/your_theme/template/my_custom_js_block/
app/design/frontend/default/your_theme/template/my_custom_js_block/extra-js.phtml
At this point you have a phtml template file that will be rendered on every page. You can add whatever javascript variables you want. I'm fond of a pattern something like
#File: app/design/frontend/default/your_theme/template/my_custom_js_block/extra-js.phtml
<?php
$h = Mage::helper('core');
$info = new stdClass();
$info->base_dir = Mage::getBaseDir();
?>
<script type="text/javascript">
var my_custom_js_block_info = <?php echo $h->jsonEncode($info); ?>;
</script>
(untested, top-of-my-head code, but should work)
