考察变量提升、函数提升、作用域。
var a = 10
function fun1(){
console.log(a) //undefined
var a = 11
}
fun1()
console.log(a)//10
var b = 20
function fun2(){
console.log(b) //20
b = 21
}
fun2()
console.log(b)//21
var c = 30
function fun3(c){
console.log(c) //undefined
c = 31
}
fun3()
console.log(c)//30
var d = 40
function fun4(d){
console.log(d) //555
d = 31
}
fun4(555)
console.log(d)//40