Functions With A Return Value

PHP Code:

function mnu() {

var thismenu=this;

var mDiv = document.createElement("div");

mDiv.getVisibility=function() { thismenu.getVisibility(this); }

alert(mDiv.getVisibility());

document.body.appendChild(mDiv);

}

mnu.prototype.getVisibility=function(div) {
if (div.style.visibility=='none' || div.style.visibility=='hidden') {
    return 'hidden'
    }
else {
    return 'visible'
    }
}


so i attach a function to mDiv, which (should) return visible or hidden, but it doesn't, i get an undefined

but when i put alert boxes in the function itself (in the if-clause), it gives me the correct values....