How to send and receive variables with Javascript?

Hi, thanks for answer... 

I'm totally numb at coding :( This code must be writting in a "execute javascript trigger" or in a js file?

Wich I can't see is where to put the function in js who send the variable and receive... and wich code is in the triggers as i do in flash

Flash:

import flash.external.ExternalInterface;
var devolucion = ExternalInterface.call("completarCurso",indice);

JS file

function completarCurso(numModActual)
{
//alert("num apartado="+ numModActual);
// do sometething

}

I just call the function from flash to send or get the variable from the js...  i can't inderstand how to do that in Storyline... that's what i don't know how to place your code