【笔记】javascript原型链继承实例

function Shape(){
 this.name = 'shape';
 this.toString = function(){
  return this.name;
 }
}

function TwoDShape(){
 this.name = '2D shape';
}
function Triangle(side,height){
 this.name = 'Triangle';
 this.side = side;
 this.height = height;
 this.getArea = function(){
  return this.side*this.height/2;
 };
}

/* inheritance */
TwoDShape.prototype = new Shape();
Triangle.prototype = new TwoDShape();

当我们对对象的prototype属性进行完全重写时，有时候会对对象constructor属性产生一定的负面影响。

所以，在我们完成相关的继承关系设定后，对这些对象的const属性进行相应的重置是一个非常好的习惯。

TwoDShape.prototype.constructor = TwoDShape;
Triangle.prototype.constructor = Triangle;

改写：

<span style="font-family:Microsoft YaHei;font-size:14px;color:#333333;">function Shape(){}

Shape.prototype.name = 'shape';
Shape.prototype.toString = function(){
 return this.name;
}

function TwoDShape(){}

TwoDShape.prototype = new Shape();
TwoDShape.prototype.constructor = TwoDShape;

TwoDShape.prototype.name = '2d shape';

function Triangle(side,height){
 this.side = side;
 this.height = height;
}

Triangle.prototype = new TwoDShape;
Triangle.prototype.constructor = Triangle;

Triangle.prototype.name = 'Triangle';
Triangle.prototype.getArea = function(){
 return this.side*this.height/2;
}</span>

再改写（引用传递而不是值传递）：

<span style="font-size:14px;color:#333333;">function Shape(){}

Shape.prototype.name = 'shape';
Shape.prototype.toString = function(){
 return this.name;
}

function TwoDShape(){}

TwoDShape.prototype = Shape.prototype;
TwoDShape.prototype.constructor = TwoDShape;

TwoDShape.prototype.name = '2d shape';

function Triangle(side,height){
 this.side = side;
 this.height = height;
}

Triangle.prototype = TwoDShape.prototype;
Triangle.prototype.constructor = Triangle;

Triangle.prototype.name = 'Triangle';
Triangle.prototype.getArea = function(){
 return this.side*this.height/2;
}</span>

虽然提高了效率，但是这样的方法有个副作用，因为是引用传递，而不是值传递，所以“父对象”中的name值受到了影响。

子对象和父对象指向的是同一个对象。所以一旦子对象对齐原型进行修改，父对象也会随即被改变。

再再改写（使用临时构造器）：

<span style="font-size:14px;color:#333333;">function Shape(){}
Shape.prototype.name = 'shape';
Shape.prototype.toString = function(){
 return this.name;
}
function TwoDShape(){}
var F = function(){}
F.prototype = Shape.prototype;
TwoDShape.prototype = new F();
TwoDShape.prototype.constructor = TwoDShape;
TwoDShape.prototype.name = '2d shape';
function Triangle(side,height){
 this.side = side;
 this.height = height;
}
F.prototype = TwoDShape.prototype;
Triangle.prototype = new F();
Triangle.prototype.constructor = Triangle;
Triangle.prototype.name = 'Triangle';
Triangle.prototype.getArea = function(){
 return this.side*this.height/2;
}</span>

虽然提高了效率，但是这样的方法有个副作用，因为是引用传递，而不是值传递，所以“父对象”中的name值受到了影响。

子对象和父对象指向的是同一个对象。所以一旦子对象对齐原型进行修改，父对象也会随即被改变。