javascript - 如何避免在没有值的对象键中未定义？

Question

我正在尝试像这样迭代一个对象:

0: Object
appointments: Array[2]
unavailables: Array[2]
0: Object
1: Object
book_datetime: "2015-10-22 02:46:23"
data: "0"
end_datetime: "2015-10-22 13:00:00"
hash: null
id: "21"
id_google_calendar: null
id_services: null
id_users_customer: null
id_users_provider: "87"
is_unavailable: "1"
notes: "Nessuna"
resource_id: "0"
start_datetime: "2015-10-22 12:00:00"
__proto__: Object
length: 2
__proto__: Array[0]
__proto__: Object
1: Object
appointments: Array[1]
unavailables: Array[0]
length: 0
__proto__: Array[0]
__proto__: Object
length: 2
__proto__: Array[0]

这样:

$.each(response, function(_, obj) 
{
        $.each(obj, function(key, val) 
        {
                if (key === 'unavailables') 
                {
                        console.log("=> ", val[0]['id']);
                }
        });
});

现在一切正常，但您如何看到第二个数组包含 unavailables length = 0 ，当循环在这里时，我得到

Cannot read property 'id' of undefined

如何避免这种情况？有没有一种方法可以检查当前键是否包含值？

Answer 1

在访问 val 之前，我可能会再做一次检查:

if (key === 'unavailables' && val && val.length) 
{
    console.log("=> ", val[0]['id']);
}