我不了解对象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

我不了解对象的可写和可配置属性.例如，在MDN中，用于 Object.prototype，在一张表中，我可以清楚地看到 Object.prototype 的可配置，可写和可枚举的属性被锁定.

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); // 999

// Example 2
var o = {};
console.log(o.testing); // 999

推荐答案

MDN所指的是 Object 本身的属性 prototype .您不能覆盖 Object.prototype 本身.如果尝试使 Object.prototype 不确定，则将失败:

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使用此方法，则在尝试将其分配给不可写的属性时会得到 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] }

您可以写入对象自己的属性，而无需更改对象的引用，并且这些属性具有单独的属性.例如，请参见:

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]] 内部属性，可防止在对象上创建新属性-如果调用 false > Object.preventExtensions ， 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，对 Object.prototype 之类的对象调用 Object.freeze 并不是一个好主意，因为可能会发生奇怪的事情:

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] } 

就像前面的示例一样，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，它也会使用原型链中的属性来检查是否可以分配该属性.有人将此语言视为语言错误，但是其他人则认为这种行为是设计使然.

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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