JavaScript: Object.prototype.__proto__

obj.__proto__

Return the parent of obj.

obj.__proto__ = parentX

Set the parent of obj to parentX.
Object.prototype.__proto__ is both a getter and a setter property. [see Getter/Setter Properties]

Tip: Object.prototype.__proto__ is not standard part of JS. It was invented by Firefox browser in 2000s, then supported by all browsers, and JS2015 spec standardize the behavior, but not part of the JavaScript language. It is not supported in some JavaScript implementations. It's not supported in TypeScript nor Deno, as of 2020-09-13

WARNING: use of the property "__proto__" may fail if the object does not have a parent, or if in the parent chain some object has a property string key "__proto__".

Better use Reflect.getPrototypeOf and Reflect.setPrototypeOf .

// set a obj's parent using __proto__
const e1 = {};
const e2 = {};
e2.__proto__ = e1;
console.log( Reflect.getPrototypeOf ( e2 ) === e1 ); // true

Get Prototype Example

console.log( 
 Reflect.getPrototypeOf ([4,6]) === [4,6].__proto__
); // true

Set Prototype Example

const aa = [4,6];
function FF () { }
aa.__proto__ = FF
console.log( Reflect.getPrototypeOf ( aa ) === FF ); // true

__proto__ Fail Example

// create a object with no parent
const bb = Object.create( null);

// find out what's the parent
console.log( bb.__proto__ === undefined); // true
// wrong

console.log( Reflect.getPrototypeOf ( bb ) === null ); // true
// correct

Here is another example, where in the parent chain a object contains a string key "__proto__".

// create a object with string key __proto__
const bb = {["__proto__"]:3};

// it has a string key __proto__
console.log( Reflect.ownKeys ( bb ) ); // [ '__proto__' ]

// key value is 3
// this shows __proto__ failed
console.log( bb.__proto__ === 3); // true

// Reflect.getPrototypeOf works
console.log( Reflect.getPrototypeOf ( bb ) === Object.prototype ); // true

// 2018-07-19 thanks to Dr. Axel Rauschmayer of 2ality http://2ality.com/2015/09/proto-es6.html for ways to create a string key property "__proto__"