Restrict user to enter only number using javascript

Code:-

<script type="text/javascript">

//Function to allow only numbers to textbox

function validate(key)

{

//getting key code of pressed key

var keycode = (key.which) ? key.which : key.keyCode;

var phn = document.getElementById('txtPhn');

//comparing pressed keycodes

if ((keycode < 48 || keycode > 57))

{

return false;

}

else

{

//Condition to check textbox contains ten numbers or not

if (phn.value.length <10)

{

return true;

}

else

{

return false;

}

}

}

</script>

<asp:TextBox ID="txtPhn" runat="server" onkeypress="return validate(event)"></asp:TextBox>