Select the first JSON array child value

I want to get the value that holds the first child of a JSON array.

This is the array :

var sms = {
    '1' : {
        'address' : '+123',
        'body' : 'SMS1'
    },
    '2' : {
        'address' : '+123',
        'body' : 'SMS2'
    },
    '3' : {
        'address' : '+123',
        'body' : 'SMS3'
    },
    '4' : {
        'address' : '+123',
        'body' : 'SMS4'
    }
};

The reason is i want to loop in it to search a string using the first childs value as it is above. Example sms.[0] = 1;