var foo = new Object();
//или 
var foo = {};

foo['bar'] = any_object;

1)
 if('bar' in foo)
2)
 if(typeof(foo['bar'] != 'undefined'))
3) 
    for(var o in foo) 
    {
   if(o == 'bar') return foo[o];
    }

var some = foo['bar'];
//или
some = foo.bar;

delete foo.bar