有關繼承

有關繼承

function Graph() {
  this.vertices = [];
  this.edges = [];
}

const g = new Graph();

g.hasOwnProperty('edges')
// true

Graph.prototype.c = 123

// 繼承父親的屬性會在 __proto__ 顯示
g.hasOwnProperty('c')
// false

g.__proto__.hasOwnProperty('c')
// true

似乎將

Dog.prototype.__proto__ = EventEmitter.prototype;

改成

Dog.prototype= EventEmitter.prototype;

Dog.prototype =new EventEmitter();

都可以跑，但這樣不是應該會蓋掉Dog的prototype，而找不到name屬性報錯嗎

var EventEmitter = require('events').EventEmitter;

function Dog(name) {
  this.name = name;
}

Dog.prototype.__proto__ = EventEmitter.prototype;
//等同於 Dog.prototype = Object.create(EventEmitter.prototype);

var simon = new Dog('simon');

simon.on('bark', function(){
  console.log(this.name + ' barked');
});

setInterval(function(){
  simon.emit('bark');
}, 500);

以上的解釋是

EventEmitter的屬性是後來用

EventEmitter.prototype.xxx="bbb"

去定義的，而不是一開始的建構子

而下面這三個，只讀的到後來用EventEmitter.prototype.xxx定義的東西

Dog.prototype.__proto__ = EventEmitter.prototype;

Dog.prototype= EventEmitter.prototype;

 Dog.prototype = Object.create(EventEmitter.prototype);

想要繼承到建構子跟後來定義的prototype只能用以下這個方法

Dog.prototype =new EventEmitter();

可看下面的範例

var EventEmitter = require('events').EventEmitter;

function Dog(name) {

  this.name = "name";
};
function DDog(name) {

  this.aabb = "ass";
};
DDog.prototype.aa="aaaaa";
Dog.prototype=new DDog();

var aaa = new Dog();
console.log(aaa.aa);

而使用Dog.prototype= DDog.prototype;也不會覆蓋原本的Dog建構子

function Dog(name) {

  this.name = "name";
};
function DDog(name) {

  this.aabb = "ass";
};
DDog.prototype.aa="aaaaa";
Dog.prototype= DDog.prototype;

var aaa = new Dog();
console.log(aaa.name);

避免將函式內的屬性設為name

function Daaog(as) {

};
console.log(Daaog.name);///沒有name但一樣會有console

其他預設屬性也不要使用，拿來命名屬性

Function.arguments
Function.arity
Function.caller
Function.displayName
Function.length
Function.name
Function.prototype

使用prototype指定屬性後要用new出新物件後才可使用

delete可用來刪除函式中的方法

delete User.save;

如果要刪除prototype加上prototype即可，其子代的該方法也被一併刪除

delete User.prototype.save;

ES5 With Object.create

var a = {a: 1}; 
// a ---> Object.prototype ---> null

var b = Object.create(a);
// b ---> a ---> Object.prototype ---> null
console.log(b.a); // 1 (inherited)

var c = Object.create(b);
// c ---> b ---> a ---> Object.prototype ---> null

var d = Object.create(null);
// d ---> null
console.log(d.hasOwnProperty); 
// undefined, because d doesn't inherit from Object.prototype

更多可參考

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Inheritance_and_the_prototype_chain