Inheritance in js

JavaScript is a class-free, object-oriented language, and as such, it uses prototypal inheritance instead of classical inheritance. 1

有几点关键的:

1.Javascript没有class的概念，在class based的语言中(Python,java,C++)，开始先定义一个class，然后再跟进这个class实例化一个object。在JS中，object是从另一个object创建（不是实例化）的。

比如:

function Vehicle(hasEngine,hasWheels)
{
    this.hasEngine = hasEngine || false;
    this.hasWheels = hasWheels || false;
}
var v = Vehicle(true,true);

这里Vehicle是一个function object. v是一个通过这个function object创建的object.

Prototype构成了一个chain. 用于实现继承。比如

function Car(make,model,hp)
{
    this.hp = hp;
    this.make = make;
    this.model = model;
}
Car.prototype = new Vehicle(true,true); //Car.prototype 指向Vehicle
Car.prototype.constructor = Car;//使得instance object的constructor指向Car
var car = new Car('audi','a6',120);
console.log(car.hasWheels) //这里输出true

这里是先创建了两个functions：Car和Vehicle, 并且把Vehicle作为Car的prototype.

2.当一个function创建时，其prototype指向新创建对象的prototype。比如:

function Foo(y) {
  this.y = y;
};

Foo.prototype.x = 10;

// and inherited method "calculate"
Foo.prototype.calculate = function (z) {
  return this.x + this.y + z;
};
var b = new Foo(20);
/*
Foo {y: 20, x: 10, calculate: function}
    y: 20
    __proto__: Foo
        calculate: function (z) {...}
        constructor: function Foo(y) {..}
        x: 10
        __proto__: Object
}
*/
var c = new Foo(30);

当前objects之间的关系是: