Maintain selection coordinates using a closure. This example using jQuery.

Box Coordinates

var boxCoord = function(endCoord) {
  var coord = endCoord;
  return function(anotherCoord) {
    return [anotherCoord, coord];
  }
}

$('body').mousedown(function(e){
  var thate = e;
  $(this).mouseup(function(e){
    var box = boxCoord([e.pageX,e.pageY])([thate.pageX,thate.pageY]);
    console.log(box);
  });
});

$('body').mousedown(function(e) { 
  $(this).mouseup(function(f) { 
    console.log([[e.pageX, e.pageY], [f.pageX, f.pageY]]); 
  }); 
});

That's far more concise. Was trying out some patterns from Crockford's JavaScript book, the good parts.

ha ha, I just noticed we are adding the mouseup event every time they mousedown. So after n mousedowns, the mouseup will be called n times.

True, to fix...

$('body').mousedown(function(e) {
  $(this).bind('mouseup.box',function(f) {
    console.log([[e.pageX, e.pageY], [f.pageX, f.pageY]]);
    $(this).unbind('mouseup.box');
  });
});

deadHorse.beat();

(function() {
  var d = {};
  $('body').bind({
    mousedown: function(e) { d.start = [e.pageX, e.pageY]; },
    mouseup: function(e) { d.end = [e.pageX, e.pageY]; console.log(d); }
  });
})();

@webdevwilson - won't arbitrarily binding both events to a dom object mean that you could - say - have a mouseup event that occurs before the mousedown event? wouldn't that mean that the console.log(d) writes out an incomplete d object?

@namlet: what's the mouseup.box for?

@azizshamim - I guess. But isn't it equally possible that the late bound mouseup could be missed completely? At least in my model the coordinates will still be captured.

@namelet - I was wondering about the mouseup.box as well.

@webdevwilson - i have no idea. i'm a javascript newbie. i asked namlet for a "how to do x" this is the result...

@azizshamim - ha ha, no one does. Perhaps the browser is slow firing the mousedown event and for some reason the mouseup is fired before the mousedown. Surely that violates a spec or something. Of course, no browser implementation would ever dare to violate a spec. Would they? Welcome to javascript. :)

mouseup.box is namespacing. That way when you unbind it, it only removes the handlers specifically registered with that namespace, mouseup.box.