ASP.NET textbox atlatma | JS

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<head runat="server">

    <script type="text/javascript" language="Javascript">
      function
atla(deger, karakter, alan)

              {if (deger.length >= karakter) { eval(alan).focus();}}
    </script>


</head>

<body>

<form id="form1" runat="server">

  <asp:TextBox ID="TextBox1" runat="server" onkeyup="atla(this.value,4,'document.form1.TextBox2');"></asp:TextBox>
  <asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>

</form>

</body>

</html>