Set height dynamic to width

by ashleycam3ron

HTML

<style>
    .wrap{
    width:300px;
    height:400px;
    border:1px solid red;
    }
    
    .child{
    width: 80%;
    border:1px solid blue;
    }
</style>

<div class="wrap">
    <div class="child"></div>
</div>

<script type="text/javascript" src="http://code.jquery.com/jquery-1.6.2.js"></script>
<script>
    var cw = $('.child').width();
    $('.child').css({
    'height': cw + 'px'
    });
</script>