http://jsfiddle.net/seSMx/3/

function add() {
   return 1 + (three || 0);
}

var three = 3;
document.getElementById('thediv').innerHTML = add();

虽然,在我看来这很难遵循.我会更进一步,并提出三个论点来补充

function add(three) {
   return 1 + (three || 0);
}

document.getElementById('thediv').innerHTML = add(3);