可枚举的意思是什么？ [英] What does enumerable mean?

问题描述

我被导向了MDN的 ..in page 当它说，for..in迭代一个对象的可枚举属性。

I was directed to MDN's for..in page when it said, "for..in Iterates over the enumerable properties of an object."

然后我去了属性页面的可用性和所有权，其中说可枚举的属性是可以由一个迭代的属性for..in loop。

Then I went to the Enumerability and ownership of properties page where it said "Enumerable properties are those which can be iterated by a for..in loop."

字典将可枚举定义为可数，但我无法真实地想象这意味着什么。我可以得到一个可枚举的例子吗？

The dictionary defines enumerable as countable, but I can't really visualize what that means. Could i get an example of something being enumerable?

推荐答案

可枚举的属性是可以包含在<$期间的一个属性c $ c> for..in 循环（或类似的属性迭代，如 Object.keys（））。

An enumerable property is one that can be included in and visited during for..in loops (or a similar iteration of properties, like Object.keys()).

如果某个属性未被识别为可枚举，则循环将忽略它在对象内。

If a property isn't identified as enumerable, the loop will ignore that it's within the object.

var obj = { key: 'val' };

console.log('toString' in obj); // true
console.log(typeof obj.toString); // "function"

for (var key in obj)
    console.log(key); // "key"

---

属性被识别为可枚举与否的 [[Enumerable]] 属性。您可以将其视为属性描述符<的一部分/ a>：

---

A property is identified as enumerable or not by its own [[Enumerable]] attribute. You can view this as part of the property's descriptor:

var descriptor = Object.getOwnPropertyDescriptor({ bar: 1 }, 'bar');

console.log(descriptor.enumerable); // true
console.log(descriptor.value);      // 1

console.log(descriptor);
// { value: 1, writable: true, enumerable: true, configurable: true }

A for..in 循环然后遍历对象的属性名称。

A for..in loop then iterates through the object's property names.

var foo = { bar: 1, baz: 2};

for (var prop in foo)
    console.log(prop); // outputs 'bar' and 'baz'

但是，只评估其声明 - console.log（prop）; 在这种情况下 - 对于那些 [[Enumerable]] 属性的属性true 。

But, only evaluates its statement – console.log(prop); in this case – for those properties whose [[Enumerable]] attribute is true.

此条件已到位，因为对象还有更多属性，尤其是来自继承：

This condition is in place because objects have many more properties, especially from inheritance:

console.log(Object.getOwnPropertyNames(Object.prototype));
// ["constructor", "toString", "toLocaleString", "valueOf", "hasOwnProperty", "isPrototypeOf", "propertyIsEnumerable", /* etc. */]

这些属性中的每一个仍然存在于对象上：

Each of these properties still exists on the object:

console.log('constructor' in foo); // true
console.log('toString' in foo);    // true
// etc.

但是，它们被<$跳过了c $ c> for..in 循环因为它们不可枚举。

But, they're skipped by the for..in loop because they aren't enumerable.

var descriptor = Object.getOwnPropertyDescriptor(Object.prototype, 'constructor');

console.log(descriptor.enumerable); // false

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