How to center align pop up div using Javascript

Question

How to align a pop up division to center of monitor/screen using javascript?

I tried using screen.width and screen.height to get center. But the division gets aligne

Answer 1

Try fixed-positioning:

#box {
  position: fixed;
  width: 40%;
  margin: 200px 30%;
}

It's only horizontally centered. Vertical will take some playing with. I have no idea how browsers act differently with the vertical alignment.