How to validate Numeric Text Box using javascrit

<Script Type = "Javascript">

function numbersonly(e)

    {

//alert('hi');

var unicode=e.charCode? e.charCode : e.keyCode;

if (unicode!=8 && unicode!=46 && unicode!=9)

{

if (unicode<48||unicode>57)

return false

}

}

How to access

<asp:TextBox ID="TextBox1" runat="server" onkeypress="return numbersonly(event);"></asp:TextBox>