为什么将 object 设置为不可扩展使其 [[prototype]] 不可变？

Question

The MDN - Object.preventExtensions pages said

This method makes the [[prototype]] of the target immutable; any [[prototype]] re-assignment will throw a TypeError . This behavior is specific to the internal [[prototype]] property, other properties of the target object will remain mutable.

And my question is:

Why does setting an object to be not extensible make its [[prototype]] immutable? ( Object.preventExtensions() , Object.seal() , Object.freeze() etc.)

Answer 1

If the internal [[prototype]] wasn't made an immutable property, you would be able to circumvent Object.preventExtensions() by exchanging the internal [[prototype]] of the object with another value using Object.setPrototypeOf() , effectively adding all the properties on the new value to the object:

 let a = {}; // a now has all the properties of Array.prototype Object.setPrototypeOf(a, Array.prototype); a.push('foo'); console.log(a); let b = Object.preventExtensions({}); // must not be able add properties to b in the same way Object.setPrototypeOf(b, Array.prototype); b.push('bar'); console.log(b);