Add multiple functions to windows onunload

get the current unload function and add your funtion to it

var fnonunload = window.onunload;
if (fnonunload)
{
fnonunload = fnonunload +"";
fnonunload = fnonunload.replace("}", "closeChildWindow();");
fnonunload = fnonunload.replace("function anonymous() {", " ");
fnonunload = fnonunload.replace("function () {", " ");
window.onunload = new Function(fnonunload);
}
else
{
fnonunload = "closeChildWindow();";
window.onunload = new Function(fnonunload);
}

here by you could dynamically add multiple onunload functions from different places in the page.