How do I get the opacity of an element using Javascript?

Question

If I have:

#em {
  opacity:0.5;
}

How do I get #em's opacity using javascript? :D

I've got troubles with the following (it returns nothing):

return document.getElementById("em").style.opacity;

Answer 1

var em = document.getElementById("em");
var  temp = window.getComputedStyle(em).getPropertyValue("opacity");

Now, the variable temp will have the value of opacity of "em".