Let’s say you have a textbox (ASP.Net control) named:
‘txtFirstName’
To be able to use the ID in your Javascript functions, you can do something like this:
var txtbox = document.getElementById(‘<%=txtFirstName.ClientID%>’);

Then, you can use the variable ‘txtBox’ throughout your Javascript function, with the normal Javascript attributes (txtbox.value, txtbox.disabled, etc)