Leading zeros (Javascript)

function _leadingZeros(n, zero_count)
{
    if(n == null || n == undefined)
        return n;

    var nstr = n.toString();
    var n_int = parseInt(nstr);
    if(isNaN(n_int))
        return n;

    while(nstr.length < zero_count)
        nstr = "0" + nstr;

    return nstr;
}