CODEWITHSUNDEEP
javascript
sgko
sgko

Reputation: 135

Object.prototype.__proto__ is not null

Why is it possible to change Object.prototype.proto from null.

Object.prototype is one of the rare objects that has no prototype, and it is not inheriting any properties (according to Flanagan)

I cant imagine how to use Object.prototype.proto in real life. Please give me an example why is it necessary to change it

Upvotes: 1

Views: 124

Answers (3)

Bergi
Bergi

Reputation: 664579

Why is it possible to change Object.prototype.__proto__ from null?

Well, it is not possible. Try it yourself in any modern browser:

> Object.setPrototypeOf(Object.prototype, Object.create(null))
Uncaught TypeError: Immutable prototype object '#<Object>' cannot have their prototype set

These immutable prototype exotic objects were introduced with ES7.

Upvotes: 1

Matthew Lagerwey
Matthew Lagerwey

Reputation: 120

Here is a whole page about Object.prototype.proto link note that it is used as a getter and setter method to expose the prototype of an object and that it has been deprecated in favor Object.getPrototypeOf and Object.setPrototypeOf. It has plenty of uses when you want to change the properties of an Objects prototype.

Upvotes: 1

T.J. Crowder
T.J. Crowder

Reputation: 1074475

I cant imagine how to use Object.prototype.__proto__ in real life

You wouldn't use it on Object.prototype itself. __proto__ is an accessor property inherited by all objects that inherit from Object.prototype. You can use it to get, or even change, their prototype (but shouldn't, keep reading). E.g.:

class X {
}
let o = {};
console.log(o.__proto__ === Object.prototype); // true
o.__proto__ = X.prototype;
console.log(o instanceof X);                   // true
console.log(o.__proto__ === Object.prototype); // false

It's provided primarily to offer compatibility with existing code on the web that used a proprietary extension to JavaScript in Mozilla's JavaScript engines. Instead of using o.__proto__, use Object.getPrototypeOf(o) and Object.setPrototypeOf(o, ...). (In fact, officially JavaScript engines aren't supposed to have __proto__ at all except when they're being used in web browsers. It's defined in an annex to the spec, not the spec itself, which only applies to web browser JavaScript engines.)

An in general, don't even use Object.setPrototypeOf. Changing the prototype of an object after it's been constructed is almost always indicative of a design problem. (And makes subsequent operations on the object very slow [in relative terms].)

Upvotes: 2

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