基于构造函数

function Circle(r) {

    this.r = r;

}

Circle.PI = 3.14159;

Circle.prototype.area = function() {

    return Circle.PI * this.r * this.r;

}

调用

var c = new Circle(1.0);

console.log(c.area())//3.14159;

类Json写法

var Circle={

    "PI":3.14159,

    "area":function(r){

        return this.PI * r * r;

    }

}; 

调用

console.log(Circle.area(1.0));//3.14159