闭包题

• 函数在执行的过程中，先从自己内部找变量；
• 如果找不到，再从创建当前函数所在的作用域去找，依此往上；
• 注意找的是变量的当前的状态

var a = 1
function fn1(){
  function fn3(){
    function fn2(){
      console.log(a)
    }
    fn2()
    var a = 4
  }
  var a = 2
  return fn3
}
var fn = fn1()
fn() //输出多少   //undefined

var a = 1
function fn1(){
  function fn3(a){
    function fn2(){
      console.log(a)
    }
    fn2()
    a = 4
  }
  var a = 2
  return fn3
}
var fn = fn1()
fn(5) //输出多少  5

var a = 1
function fn1(){
  function fn3(){
    var a = arguments[0]  //相当于
    function fn2(){
      console.log(a)
    }
    fn2()
    a = 4
  }
  var a = 2
  return fn3
}
var fn = fn1()
fn(5) //输出多少