js 闭包

function a() {
    //函数外访问不到
    var s = 3;
   //使外部能够获得s变量的值 
    this.f = function () {
        return s
    }
    //改变s的值
    this.add=function(){
        s++
    }
}

var t = new a()
t.add()
console.log(t.f())