Resize and position Javascript popup

In many of my project i came across the situation where i had to open a popup with a particular size and position the popup in the middle of the window. I was sending the height and width parameter in the window.open() funtion parameter. But many times i had to add some extra controls in that page which needs the increase of the popup size. I was problemetic for me to goback to the parent page and find the javascript which is opening the popup and change.
So i decided to control the size from the popup itself. And i used
window.resizeTo(width,height) function for that purpose and called it on body onLoad event.
But what about positioning the popup?
For both the pubpose to solve i wrote a function resizeAndPositionWindow() which takes height and width as parameter and positions the popup in the middle of the window. Below is the function...

function resizeandPositionWindow(width,height)

{

window.resizeTo(width,height);

var scrW = 1024, scrH = 768;

var popW=width,popH=height;

if (document.all document.layers) {

scrW = screen.availWidth;

scrH = screen.availHeight;

}

else

{

try

{

scrW = window.screen.availWidth;

scrH = screen.availHeight;

}

catch(e){}

}

if(document.all)

{

popW=document.body.clientWidth;

}

else

{

if(document.layers)

{

popW=window.innerWidth;

popH=window.innerHeight;

}

else

{

try

{

popW=window.innerWidth;

popH=window.innerHeight;

}

catch(e){alert('Exception');}

}

}

var x,y;

x=(scrW-popW)/2-13;

y=(scrH-popH)/2;

window.moveTo(x,y);

}

By default it is assumed that the screen resolution of the user is 1024X768. Then it determines the actual height.
I used the above funtion to make a function which will take a int parameter as gap between the popup and computer screen and will make the popup which will have that length in top,bottom,left and right.
function resizeAccordingToScreenSize(gap)
{
var scrW = 1024, scrH = 768;
if (document.all document.layers) {
scrW = screen.availWidth;
scrH = screen.availHeight;
}
else
{
try
{
scrW = window.screen.availWidth;
scrH = screen.availHeight;
}
catch(e){}
}
var gapPx=0;
try
{
gapPx=parseInt(gap)*2;
}
catch(e){alert("Wrong value for gap.")}
resizeAndPositionWindow(scrW-gapPx,scrH-gapPx);
}