Using Javascript in an aspx page

I have a .net text field and a .net button on a form.

When the user leaves the text field server side VB.net code fires to check if anything has been entered and if it has, enables the button.

The problem with this is that it has to do the round trip to the server.

How would I use javascript to do this in a .net page ?
Can I control a .net textbox with javascript ?

TIA

Yes, you can use JS to perform client-side events. .Net server controls come just as html to the browser. Its only when it goes back that the .ASP application builds the page and the controls and compares them.