prototype與__proto__

對象的 prototype 屬性的方法、屬性爲對象所屬的那一「類」所共有。對象原型鏈經過 __proto__ 屬性向上尋找。
爲 __proto__ 指定 null 以外的原始類型(Number, String, Boolean, undefined, Symbol)值是無效的。
經過構造函數或者 {} 方式建立的對象的 prototype 屬性默認爲 undefined

{} 的繼承

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

若是沒有明確指定，那麼 __proto__ 默認爲 Object.prototype，而Object.prototype 自身也有 __proto__ ，值爲 null，是原型鏈的終點。

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.

構造函數模式的繼承

// 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
  
);

若是沒有明確指定，經過構造函數建立的對象的 __proto__ 屬性值爲構造函數的 prototype 屬性。