You can do it like this. Add client of your control in one hidden field and then access it like;
<asp:hiddenfield id="hdnNameTextBox" runat="server" xmlns:asp="#unknown" />
void FormView1_DataBound(object sender, EventArgs e)
{
hdnNameTextBox.Value = ((TextBox)FormView1.FindControl("TextBoxId")).ClientID;
}
Now use the hidden fields value.
var id = document.getElementById('hdnNameTextBox').value;
var txt = document.getElementById(id);
Anurag