<appSettings>
  <add key="var1" value="123456"/>
</appSettings>

<script type="text/javascript">
  function doSomething() {"<%= System.Configuration.ConfigurationManager.AppSettings['var1'].ToString(); %>"
};
</script>

<script type="text/javascript">
var var1 = '<%=System.Configuration.ConfigurationManager.AppSettings["var1"].ToString();%>';
</script>