Changing width using jQuery .css() and em

Question

I need to change the width of an div using jQuery.

When I use the following, it works:

$('#footer').css('width', '100%');
$('#footer').css('width', '-=239');

When I use this, it works:

$('#footer').css('width', '100%');
$('#footer').css('width', '-=239px');

But when I use this it doesn't do anything:

$('#footer').css('width', '100%');
$('#footer').css('width', '-=21em');

Is there a way to make jQuery work with em? or to calculate em to px and set a variable for example and subtract that value?

edit: thanks everybody for correcting my spelling and code!

Answer 1

i tried it and it is working i think you should check your code i have checked it in all browser and it is working fine px and em both working well you can use this code put this code in a dummy page and u can see this works

<style>
#footer{
background: black;
height:50px;
}
</style>
<script type="text/javascript">
$(function(){
    $('#footer').css('width', '100%');
    $('#footer').css('width', '-=210em');
});
</script>
<div id="footer"></div>