How to determine which button (back or forward) was pressed in the browser?

Question

JavaScript

How to determine which button (back or forward) was pressed in the browser?
Event handler:

addEventListener("popstate",function(e){
    alert('yeees!');
},false);

This method handles the back and forth button press event, but does not let you know which button was pressed.
How can I determine the pressing of the buttons back or forward separately?