JS: Object.prototype.__proto__

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

obj.__proto__ → is equivalent to Object.getPrototypeOf ( obj )

[see JS: Object.getPrototypeOf]

obj.__proto__ = proto → is equivalent to Object.setPrototypeOf ( obj, proto )

[see JS: Object.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

Note: it's recommend you NEVER change a object's parent. Due to how JavaScript is implemented, changing a object's parent causes major slow down.

Reference

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