How to ensure plugin menu items are correctly greyed out or not (enabled / disabled) ( javascript, Adobe Bridge)

I'm making some Adobe Bridge (CS 5.1) plugins.

My problem is that I cannot figure out how to ensure that their respective menu items will be greyed out unless the user has selected valid items for the script.
I can reach the desired state by coding something like

function greyOutMenu () {
    var doc = application.document;
    var these = [];
    these = doc.selections;
    menuItem.enabled = true;
    if ( these.length < 1 ) {
        menuItem.enabled = false;
        return;
    }
    for ( var i in these ) {
        if ( these[i] /* is invalid */ ) { menuItem.enabled = false;
        return;
    }
}

but how do I get this check to run directly when the menu is opened? If I use

myMenu.onSelect = greyOutMenu();

It just runs the check on startup instead of when the menu is opened!

Okay, I figured out what was wrong. I have changed it to...

function greyOutMenu () {
    var doc = app.document;
    var here = doc.presentationPath;
    var thisFolder = Folder ( here );

    if ( decodeURI ( thisFolder.name ) === "correct folder name" ) { menuItem.enabled = true; }
    else { menuItem.enabled = false; }
    if (!app.document.selectionsLength > 0 ) { menuItem.enabled = false; }
}
menuItem.onDisplay = greyOutMenu;

I could have sworn that I had already tried menuItem.onDisplay, but I must have made a syntax error.

Also, In my case it is enough to be in the right folder and have something selected, as the files are added directly by a camera. More complex checks are instead added to the function itself, to prevent stuttering every time the menu is opened.