how to assign a user variable with javascript?

I have defined a user variable named v_current with initial value of 3.

On first slide, I've put a caption containing "Scenario: $$v_current$$".

Also on first slide, I have defined "On Enter" action to execute javascript. The Script_Window contains:

var objCp = document.Captivate;
objCp.cpEISetValue('v_current','8');

When the quiz is played, the caption says: "Scenario: 3" so v_current is not updated with javascript, why?

By the way, using Advanced Actions works fine and updates the variable, but I need javascript in this case.