somma elementi pari in lista bidirezionale

function somma_pari(l)
{
    var prev,soma=0;
    if(l==null){return somma;}
    prev=l;
    do
    { if(prev.x%2===0)
        { somma=somma+prev.x;prev=prev.succ;}
        else
        {prev=prev.succ;}
    } while(prev!=null);
    return somma;
}