JS: Object.prototype.__proto__

Object.prototype.__proto__ is both a getter and a setter property. [see JS: Getter/Setter Properties]

You can use it to get or set a object's parent object, like this:

const e1 = {};
const e2 = {};
e2.__proto__ = e1;

console.log ( Reflect.getPrototypeOf ( e2 ) === e1 ); // true

However, if the object does not have a parent (example: parent is null), or if in the parent chain some object has a property string key "__proto__", then it'll fail.

Use Reflect.getPrototypeOf and Reflect.setPrototypeOf instead.

[see JS: Get/Set Prototype]

[see JS: Reflect.getPrototypeOf]

[see JS: Reflect.setPrototypeOf]

Get Prototype Example

const arr = [4,6];

console.log (
 Object.getPrototypeOf (arr)  === arr.__proto__
); // true

console.log (
 Object.getPrototypeOf (arr)  === arr["__proto__"]
); // true

Set Prototype Example

const aa = [4,6];

function FF () { }

// set a object's parent
console.log(
    aa.__proto__ = FF
); // [Function: FF]

console.log (
    Object.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's 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__"

Reference

ECMAScript® 2016 Language Specification#sec-additional-properties-of-the-object.prototype-object