我不理解Objects的可写和可配置属性属性 [英] I don't understand writable and configurable property attributes of Objects

问题描述

我不理解Objects的Writable和Configurable属性。例如，在 Object.prototype <的MDN中/ a>，有一个表格，我可以清楚地看到 Object.prototype 的可配置，可写和可枚举的属性属性被锁定。

I don't understand the Writable and Configurable attributes of Objects. For example, in the MDN for Object.prototype, there is a table where I can clearly see that Configurable, Writable and Enumerable Property Attributes of Object.prototype are locked.

但是，我可以编写和扩展Object.prototype，例如使用以下代码：

However, I can write and extend Object.prototype, for example with the following code:

// Example 1
Object.prototype.testing=999;
console.log(Object.testing); // 9999

// Example 2
var o = {};
console.log(o.testing); // 9999

推荐答案

MDN所指的是属性原型 对象本身。您不能覆盖 Object.prototype 本身。如果您尝试使 Object.prototype 未定义，那将失败：

What the MDN is referring to is the property prototype of Object itself. You cannot overwrite Object.prototype itself. If you try and make Object.prototype undefined, that will fail:

Object.prototype = 1;
console.log(Object.prototype); // [object Object]

如果您在严格模式下尝试此操作，您将获得<$ c尝试分配给不可写属性时$ c> TypeError ：

If you try this in strict mode, you will get a TypeError upon attempting to assign to a non-writable property:

'use strict';
Object.prototype = 1; // TypeError: Cannot assign to read only property 'prototype' of function Object() { [native code] }

您可以在不更改对象引用的内容的情况下写入对象自己的属性，并且这些属性具有单独的属性。例如，请看：

You can write to an object's own properties without changing what the object's reference is, and those have separate attributes. For example, see this:

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

console.log(descriptor.writable); // true
console.log(descriptor.enumerable); // false
console.log(descriptor.configurable); // true

有一个单独的 [[Extensible]] 阻止在对象上创建新属性的内部属性 - 如果调用 Object.preventExtensions，则设置为 false code>， Object.seal 或 Object.freeze 。

There is a separate [[Extensible]] internal property that prevents the creation of new properties on an object -- this is set to false if you call Object.preventExtensions, Object.seal or Object.freeze.

请注意，在对象上调用 Object.freeze 不是一个好主意.prototype ，因为真的很奇怪：

Note that it's not a good idea to call Object.freeze on something like Object.prototype, as really weird things can happen:

Object.freeze(Object.prototype);
var building = {};
building.name = 'Alcove Apartments';
building.constructor = 'Meriton Apartments Pty Ltd';
console.log(building.constructor); // function Object() { [native code] } 

就像前面的例子一样，它也会在严格模式下抛出 TypeError 。

Just like the previous example, it will also throw a TypeError in strict mode.

基本上，即使它是对象本身的属性，它也是使用原型链中的属性来检查它是否可以分配属性。一些人用语言认为是一个错误但是其他人认为这种行为是设计的。

Basically, even though it would be a property on the object itself, it uses the attributes from the prototype chain to check whether or not it can assign the property. This has been considered as a mistake in the language by some people, however others consider this behaviour to be by design.

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