衍生內建建構器的子類別 subclass

Subclassing a built-in 的兩個障礙:

Obstacle 1 帶有內部特性的實體

function Super(x, y){
 this.x = x;
 this.y = y;
}

function Sub(x, y ,z){
 Super.call(this, x, y);
 this.z = z;
}

內建建構器會忽略傳入傳入作為this的子實體

[[Primitive Value]]
valueOf() -> toString()

	內部實體特性
包裹器建構器
Boolean	[[PrimitiveValue]]
Number	[[PrimitiveValue]]
String	[[PrimitiveValue]], [[GetOwnProperty]], length
其他建構器
Array	[[DefineOwnProperty]]
Date	[[PrimitiveValue]]
Function	[[Call]]
RegExp	[[Match]]

• Error, Object 沒有內部特性

解決方式

function copyOwnPropertiesFrom(targetObj, source){
    Object.getOwnPropertyNames(source).forEach(function(propKey){
      var propDesc = Object.getOwnPropertyDescriptor(source, propKey);
      Object.defineProperty(targetObj, propKey, propDesc);
  });
  return targetObj;
}


function MyAry(/*arguments*/){
 var ary = [];
 Array.prototype.push.apply(ary, arguments);
 copyOwnPropertiesFrom(ary, MyAry.methods);
 return ary;
}

MyAry.methods = {
  get size(){
    var size = 0;
    for (var i = 0; i < this.length; i++){
      if(i in this) size++;
    }
    return size;
  }
}

var a = new MyAry('aaa','bbb');
a.length = 5 
a.length// 5
a.size // 2

Obstacle 2 無法被當作函式呼叫的建構器

Ex. Error 無法用上述方式subclass他們

解決方式

1. 子建構器Sub 內創建新的超實體Super 並複製自有特性到子實體Sub

function NewError(){
 var superInstance = Error.appy(null, [arguments]);
 copyOwnPropertiesFrom(this, superInstance);
}

NewError.prototype = Object.create(Error.prototype);
NewError.prototype.constructor = NewError;

2. 代理 delegation

   function MyAry(/*arguments*/){
    this.ary = [];
    Array.prototype.push.apply(this.ary, arguments);
   }
   
   
   Object.defineProperties(MyAry.prototype, {
     size: {
       get: function(){
         var size = 0;
         for (var i = 0; i < this.length; i++){
           if(i in this) size++;
         }
         return size;
       }
     },
   });
   
   //限制: 不能使用[]取值 需用此方式設定get set達成這個功能
   MyAry.prototype.get = function(index){
     return this.ary[index];
   }
   MyAry.prototype.set = function(index, value){
     return this.ary[index] = value;
   }
   
   //轉移所有Array.prototype的方法
   
   Object.getOwnPropertyNames(Array.prototype).forEach(function(propKey){
     MyAry.prototype[propKey] = function(){
     return Array.prototype[propKey].apply(this.ary, arguments);
         }
   });