Object.prototype.__proto__

var shape = {},
  circle = new Circle();

//   
shape.__proto__ = circle;
//   
console.log(shape.__proto__ === circle); // true

var noProto = Object.create(null);

console.log(typeof noProto.__proto__); // undefined
console.log(Object.getPrototypeOf(noProto)); // null

noProto.__proto__ = 17;

console.log(noProto.__proto__); // 17
console.log(Object.getPrototypeOf(noProto)); // null

var protoHidden = {};
Object.defineProperty(protoHidden, "__proto__", {
  value: 42,
  writable: true,
  configurable: true,
  enumerable: true,
});

console.log(protoHidden.__proto__); // 42
console.log(Object.getPrototypeOf(protoHidden) === Object.prototype); // true

//  ,   
function Employee() {
  /*   */
}

//    Employee
var fred = new Employee();

//   
fred.__proto__ === Employee.prototype; // true

function Cow() {
  /*   */
}

//  __proto__  
fred.__proto__ = Cow.prototype;

var obj = {};
Object.preventExtensions(obj);

obj.__proto__ = {}; //  TypeError

var b = {};

Object.prototype.__proto__ = Object.create(
  null, // [[Prototype]]
  {
    hi: {
      value: function () {
        alert("hi");
      },
    },
  },
);

b.hi();