旧的Firefox支持 __ count __ property。较新的环境支持ES5的 Object.keys 。对于较旧的环境，我们必须回退到迭代对象并手动计数（呃！）：

Old Firefox supports the __count__ property. Newer environments support ES5's Object.keys. For older environments we have to fallback to just iterating over the object and counting manually (ugh!):

function count(obj) {

    if (obj.__count__ !== undefined) { // Old FF
        return obj.__count__;
    }

    if (Object.keys) { // ES5 
        return Object.keys(obj).length;
    }

    // Everything else:

    var c = 0, p;
    for (p in obj) {
        if (obj.hasOwnProperty(p)) {
            c += 1;
        }
    }

    return c;

}