如何在没有值的对象键中避免未定义？

Question

I'm trying to iterate an object like this:

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]

in this way:

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

Now all working fine but how you can see the second array contain unavailables length = 0 , when the loop is on here I get

Cannot read property 'id' of undefined

How can avoid this? There is a method that check if the current key has contain value or not?

Answer 1

I'd probably go for another check before accessing val :

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

Answer 2

You can try the following. If val[0] doesn't exist, it will be replaced with an empty hash, that will respond to ['id'] with undefined (but avoid errors.)

(val[0] || {})['id']

Answer 3

Try checking if val[0]['id']) exists:

if (key === 'unavailables' && val[0]['id'])){

} 

Answer 4

Check if the length property of val as such

if (val.length > 0) {
   console.log("=> ", val[0]['id']);
} else {
   // contains no value
}

Answer 5

The existing answers work, but I'd like to point out the fact that your approach is not optimal: you iterate over all properties of all obj s.

In fact you don't need to do that; you can access the unavailables property directly. Your entire code could therefore be reduced to this:

$.each(response, function(_, obj) 
{
  var val = obj.unavailables;

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