# this
1 

function fn2(){
    console.log(this.n)
    var n='n'
    this.n=10
    console.log(n)
}
var obj={fn2:fn2, n:1}
fn2()
obj.fn2()
console.log(obj.n, window.n)

//undefined 'n'
//1 'n'
//10 10

2 

function f(){console.log(this)}

var obj={
    fn: (function(){
        return this.f
    })(),
    f: function(){console.log(this)}
}
f()
obj.f()
obj.fn()
// window obj obj

3

var n = 10
var obj1={
    n:1,
    f:function(){this.n++; n=this.n++}
}

obj1.f() 
console.log(n) // 2
console.log(obj1.n) // 2
window.setTimeout(obj1.f, 1000)

//2
//3

4

console.log(getA)
if('a' in window){
     var a = ''
    function getA(a){
        a = a||this.a
        console.log(this.a)
    }
    getA(a)
}

5

var a=2
var obj1 = {
    a:1,
    fn1: (function(a){
        this.a = a
        a++
        return function(){
            this.a = a++
            console.log(a)
        }

    })(a)
}
obj1.fn1() // 4
var fn1 = obj1.fn1 
fn1() // window.a = 4,a=5

6

var c=3
function getC(){
    this.c++
    return function (){
        c=this.c*2
        console.log(c)
    }
}
var obj3={
    c: 2,
    getC:(function(){
        this.c -= 1
        return this.getC
    })()
}
getC() // window.c = 3
obj3.getC() // obj3.c=3
var f3=obj3.getC
f3() // window.c=4
console.log(window.c) // 4
console.log(obj3.c) // 3


for (var i = 1; i <= 5; i++) {

  setTimeout( function timer() {

      console.log(i);

  }, 1000 );

}

上面的代码会输出什么？怎么改动上述代码，使其依次输出1、2、3、4、5 并说明原因


理论题：
谈谈什么是闭包