How do I control the alpha of an object, via jscript?

Hi.
How can I change the opacity (alpha) of an object, via JavaScript?
The Transform for that object is in variable tgt.
But I haven't found the proper way to access its color.alpha - I am not even sure that is the way to do it.
The object is already employing the Transparent Diffuse mode, so I can actually change the alpha channel manually and it displays beautifully on the screen.
Now, I need to control that same alpha channel, just in JavaScript, instead of manually.

Thanks.

 

renderer.material.color.a = x

Where x is between 0 and 1.

--Eric

 

Thanks a lot!