如何用javascript调整div高度

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title> new document </title>
    <meta name="generator" content="editplus" />
    <meta name="author" content="Gao YiXiang" />
    <meta name="email" content="[email protected]" />
    <meta name="keywords" content="javascript dhtml dom" />
    <meta name="description" content="I love web development." />
</head>
<body>
    <input type="button" id="btnAdd" value="Add" />
    <div id="divText" style="width: 100px; height: 100px; background-color: blue;">
    </div>
    <script type="text/javascript">
    <!--
btnAdd.onclick = function()
{
    divText.style.width = parseInt(divText.style.width) + 50;
    divText.style.height = parseInt(divText.style.height) + 50;
};
    //-->
    </script>
</body>
</html>