Node.js 类

Javascript的类都声明为函数:

function Shape () {
    this.X = 0;
    this.Y = 0;
    this.move = function (x, y) {
        this.X = x;
        this.Y = y;
    }
    this.distance_from_origin = function () {
        return Math.sqrt(this.X*this.X + this.Y*this.Y);
    }
}
var s = new Shape();
s.move(10, 10);
console.log(s.distance_from_origin());

你可以随时向你的类中添加任意数量的属性和方法:

var s = new Shape(15, 35);
s.FillColour = "red";

声明类的函数是它的构造函数！

原型和继承

默认情况下，JavaScript中的所有对象都有一个原型对象。原型对象是它们继承属性和方法的机制。以下代码显示如何使用原型创建继承。更改Shape类，以便所有继承对象也获得X和Y属性，以及你声明的方法:

function Shape () {
}
Shape.prototype.X = 0;
Shape.prototype.Y = 0;
Shape.prototype.move = function (x, y) {
     this.X = x;
     this.Y = y;
}
Shape.prototype.distance_from_origin = function () {
     return Math.sqrt(this.X*this.X + this.Y*this.Y);
}
Shape.prototype.area = function () {
     throw new Error("I don"t have a form yet");
}
var s = new Shape();
s.move(10, 10);
console.log(s.distance_from_origin());
function Square() {
}
Square.prototype = new Shape();
Square.prototype.__proto__ = Shape.prototype;
Square.prototype.Width = 0;
Square.prototype.area = function () {
   return this.Width * this.Width;
}
var sq = new Square();
sq.move(-5, -5);
sq.Width = 5;
console.log(sq.area());
console.log(sq.distance_from_origin());

上面的代码生成以下结果。

你可以进一步扩展一个新的类叫Rectangle，继承自Square类:

function Rectangle () {
}
Rectangle.prototype = new Square();
Rectangle.prototype.__proto__ = Square.prototype;
Rectangle.prototype.Height = 0;
Rectangle.prototype.area = function () {
   return this.Width * this.Height;
}
var re = new Rectangle();
re.move(25, 25);
re.Width = 10;
re.Height = 5;
console.log(re.area());
console.log(re.distance_from_origin());

我们可以使用运算符instanceof来检查继承。

console.log(sq instanceof Square);      // true
console.log(sq instanceof Shape);       // true
console.log(sq instanceof Rectangle);   // false
console.log(re instanceof Rectangle);   // true
console.log(sq instanceof Square);      // true
console.log(sq instanceof Shape);       // true
console.log(sq instanceof Date);        // false