for in 循环的输出顺序问题

           var data = {
                '4': 'first',
                '3': 'second',
                '2': 'third',
                '1': 'fourth'
            };
            for (var i in data) {
                console.log(i + "  " + data[i])
            }

IE11, chrome31, firefox23的打印如下：

1  fourth
2  third
3  second
4  first

var obj = {
  "first":"first",
   "zoo":"zoo",
  "2":"2",
  "34":"34",
  "1":"1",
  "second":"second"
};
for (var i in obj) { console.log(i); };

IE11, chrome31, firefox23的打印如下：

1
2
34
first
zoo
second

根据stackoverflow给出的答案：

Currently all major browsers loop over the properties of an object in the order in which they were defined. Chrome does this as well, except for a couple cases. [...] This behavior is explicitly left undefined by the ECMAScript specification. In ECMA-262, section 12.6.4:

The mechanics of enumerating the properties ... is implementation dependent.

However, specification is quite different from implementation. All modern implementations of ECMAScript iterate through object properties in the order in which they were defined. Because of this the Chrome team has deemed this to be a bug and will be fixing it.

All browsers respect definition order with the exception of Chrome and Opera which do for every non-numerical property name. In these two browsers the properties are pulled in-order ahead of the first non-numerical property (this is has to do with how they implement arrays). The order is the same for Object.keys as well.

事实上，它不一定根据定义时的顺数输出，所有浏览器的最新版本现在都按chrome执行，先把当中的非负整数键提出来，排序好输出，然后将剩下的定义时的顺序输出。由于这个奇葩的设定，让avalon的ms-with对象排序不按预期输出了。只能强制用户不要以纯数字定义键名：

var obj = {
  "first":"first",
   "zoo":"zoo",
  "2a":"2",
  "34u":"34",
  "1l":"1",
  "second":"second"
};
for (var i in obj) { console.log(i+" "+obj[i]); };

IE11, chrome31, firefox23的打印如下：

first first
zoo zoo
2a 2
34u 34
1l 1
second second