Javascript get text value from nopcommerce:simpletextbox

Hi all,

I'm trying to retrieve the value of a nopCommerce:SimpleTextBox at runtime using javascript can anyone help me to do this please?

i have tried the following code with no luck

var source = document.getElementById('<%=MyTextBox.ClientID%>').value;

any ideas?

thanksin advance,

Matt.

var source = document.getElementById('<%=MyTextBox.ClientID%>' + "_txtValue").value;