node.js - Nodejs 属性顺序保证

Question

注意:我使用的是 Nodejs，它可能与普通 ECMAscript 的标准有细微的差别，也可能没有。

我一直听说，当使用 for-each 循环迭代对象的属性时，我不应该指望属性具有相同的顺序。 (尽管在实践中我从未见过以不同顺序迭代对象的情况)。在生产中，我们发现了一个拼写错误，即使用覆盖的属性创建了对象。

var obj = {
  a: 'a property',
  b: 'another property',
  c: 'yet another property',
  a: 'woah we have another a?'
}

在 Nodejs 中，我是否保证第二个包含字符串 'woah we had another a?' 的属性将始终隐藏第一个包含字符串 “属性”？

Answer 1

(even though in practice I have never seen a case where the objects were iterated over in a different order) The following should give you a different order in V8 atleast.

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

正如所讨论的here

ECMA-262 does not specify enumeration order. The de facto standard is to match insertion order, which V8 also does, but with one exception:

V8 gives no guarantees on the enumeration order for array indices (i.e., a property name that can be parsed as a 32-bit unsigned integer).

Remembering the insertion order for array indices would incur significant memory overhead.

虽然上面说没有指定枚举顺序，但那是在对象创建之后。我认为我们可以安全地假设插入顺序应该保持一致，因为任何引擎都没有必要这样做并改变插入顺序。

var obj = {
  "first":"first",
  "2":"2",
  "34":"34",
  "1":"1",
  "second":"second",
  2: "two"
};
// gives result
{ '1': '1',
  '2': 'two',
  '34': '34',
  first: 'first',
  second: 'second' }