prototype与__proto__

对象的 prototype 属性的方法、属性为对象所属的那一“类”所共有。对象原型链通过 __proto__ 属性向上寻找。
为 __proto__ 指定 null 之外的原始类型(Number, String, Boolean, undefined, Symbol)值是无效的。
通过构造函数或者 {} 方式创建的对象的 prototype 属性默认为 undefined

{} 的继承

var a = {
  x: 10,
  calculate: function (z) {
    return this.x + this.y + z;
  }
};
 
var b = {
  y: 20,
  __proto__: a
};
 
var c = {
  y: 30,
  __proto__: a
};
 
// call the inherited method
b.calculate(30); // 60
c.calculate(40); // 80

如果没有明确指定，那么 __proto__ 默认为 Object.prototype，而Object.prototype 自身也有 __proto__ ，值为 null，是原型链的终点。

If a prototype is not specified for an object explicitly, then the default value for __proto__ is taken — Object.prototype. Object Object.prototype itself also has a __proto__, which is the final link of a chain and is set to null.

构造函数模式的继承

// a constructor function
function Foo(y) {
  // which may create objects
  // by specified pattern: they have after
  // creation own "y" property
  this.y = y;
}
 
// also "Foo.prototype" stores reference
// to the prototype of newly created objects,
// so we may use it to define shared/inherited
// properties or methods, so the same as in
// previous example we have:
 
// inherited property "x"
Foo.prototype.x = 10;
 
// and inherited method "calculate"
Foo.prototype.calculate = function (z) {
  return this.x + this.y + z;
};
 
// now create our "b" and "c"
// objects using "pattern" Foo
var b = new Foo(20);
var c = new Foo(30);
 
// call the inherited method
b.calculate(30); // 60
c.calculate(40); // 80
 
// let's show that we reference
// properties we expect
 
console.log(

  b.__proto__ === Foo.prototype, // true
  c.__proto__ === Foo.prototype, // true
 
  // also "Foo.prototype" automatically creates
  // a special property "constructor", which is a
  // reference to the constructor function itself;
  // instances "b" and "c" may found it via
  // delegation and use to check their constructor
 
  b.constructor === Foo, // true
  c.constructor === Foo, // true
  Foo.prototype.constructor === Foo, // true
 
  b.calculate === b.__proto__.calculate, // true
  b.__proto__.calculate === Foo.prototype.calculate // true
  
);

如果没有明确指定，通过构造函数创建的对象的 __proto__ 属性值为构造函数的 prototype 属性。