JS: Object.prototype.__proto__ ❌

The Property __proto__

obj.__proto__

Return the parent of obj.

Object.prototype.__proto__ is both a getter and a setter property. 〔see Getter Setter Properties〕

🛑 WARNING: Object.prototype.__proto__ is not standard part of JS. It was invented by Firefox browser in 2000s, then supported by all browsers, and ECMAScript 2015 standardize the behavior, but not part of the JavaScript language. Use of the property "__proto__" fails if the object does not have a parent, or if the key exist in parent chain. Not supported in deno nor TypeScript

🟢 TIP: better is Reflect.getPrototypeOf and Reflect.setPrototypeOf .

// __proto__ is not supported in deno and other js engines
console.log(([4, 6].__proto__) === undefined);
// true
// in deno

Set __proto__

obj.__proto__ = xParent

Set the parent of obj to xParent.

// set a obj's parent using __proto__
const aa = {};
const bb = {};
bb.__proto__ = aa;
console.log((Reflect.getPrototypeOf(bb) === aa) === false);
// true
// in deno

Example of __proto__ Failure

// create two objects
const aa = Object.create(null);
const bb = Object.create(null);

// set parent
Reflect.setPrototypeOf(bb, aa);

// verify the parent
console.log(Reflect.getPrototypeOf(bb) === aa);
// true

// property __proto__ fails
console.log((bb.__proto__ === aa) === false);
// true

// value is undefined
console.log(bb.__proto__ === undefined);
// true

What is the difference between getPrototypeOf vs .__proto__ Property

• __proto__ may fail when a object in its parent chain has a string key "__proto__".
• __proto__ fails when object does not have a parent.
• __proto__ is not supported in Deno, TypeScript.
• __proto__ is not standard JavaScript, but is mentioned in appendix of JavaScript spec.
• __proto__ works via inheritance over prototype object Object.prototype.__proto__
• Reflect.getPrototypeOf and Reflect.setPrototypeOf always work, and are in JavaScript spec.