javascript - 继承 toString()

Question

我有一个方法toString()，它不是从Shape继承的。为什么？

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;};
}

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

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

var my = new Triangle(5, 10);

document.write("my getarea: " + my.getArea() + "my name is: " + my.toString()  + "<br>");​

Demo jsFiddle.

Answer 1

Triangle 的原型(prototype)必须是 Shape 以便它继承其方法:

Triangle.prototype = new Shape();

更具体地说，因为您有多个继承级别:

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

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

也就是说，TwoDShape继承了Shape，而Triangle继承了TwoDShape。

一般来说，如果 Foo 继承 Bar，您将拥有:

Foo.prototype = new Bar(); // inherit Bar
Foo.prototype.constructor = Foo; // Fix constructor which now points to Bar

<强> DEMO 。

引用文献:

• MDN: Introduction to Object-Oriented JavaScript: Inheritance
• OOP in JavaScript: Inheritance