how do I tell javascript what type of data I'm passing to a function?

[CMYK]

I have a function that goes like

function collapse(div) {
    var currentdiv = document.getElementById(div);
    currentdiv.style.zIndex = 3;
}

and code from the html document calls that function like

<a onclick="collapse('pictures')">pictures</a>

where pictures is the name of a <div> later on that had the id of pictures. Anyway, how can I tell Javascript that I'm passing a string so that the document.getELementById part will work? For example, the code will work if I write in document.getElementById("pictures");.