Javascript函数嵌套作用域访问

Question

您能向我解释为什么我无法访问db.collection（'mains'）范围之外的if块生成的值。在全局范围内存储结果的最佳方法是什么？

    var foo = function(){
        let res = [];
        db.collection('mains').find().toArray((error, result) => {
            if (error) throw error;
            result.forEach((result) => {
                session.pingHost(result.ip, (error, target) => {
                    if(error){
                        console.log(`Can't contact: ${target} timeStamp: ${getDateTime(true)}`);
                        res.push(target);
                    } else {
                        console.log(`${target}: OK tag: ${getDateTime(false)}`);
                        res.push(target);
                    };
                });
            });
        });
        return res;
    };

    console.log(foo());