webveuje/js/kejian/yubianyilx.md

# js info
### 预编译
链接：
*  https://juejin.cn/post/6916046341131599879
*  https://juejin.cn/post/6896445621273083912
demo1:
```
function f() {
    console.log(aa); // undefined
    var aa = 5;
    console.log(aa); // 5
}
f();
```
demo2:
```
var scope = 'global';
function t() {
    console.log(scope); // undefined
    var scope = 'local';
    console.log(scope); // local
}
t();
console.log(scope); // global

```

demo3:
```
console.log(b) //undefined 变量提升
var b =2; 
```

demo4
```
console.log(foo)
function foo(){
    
}
var foo =1
```


### 作用域

demo1:
```
function zxxFn () {
    zxxs = 2
}
var zxx = 1
console.log(window.zxx) // 1
// console.log(zxxs) 未定义
zxxFn() // 执行后zxxs成为全局变量
console.log(zxxs) // 2

```

```
var zxx = 'zxx is a good girl'
    function zxxFn () {
        console.log(zxx) 
        var zxx = 'zxx is a great girl'
        console.log(zxx)
        zxx = 'very good'
        console.log(zxx)
    }
    console.log(zxx)
    zxxFn()
    console.log(zxx)
    // 依次输出 1. zxx is a good girl 2. undefined 3. zxx is a great girl
    // 4. very good 5.zxx is a good girl
```

```
zxxAge = 'zxx is 18 years old'
function zxxFn () {
    console.log(zxxAge)
    var zxxAge = 'zxx is 8 years old'
    var zxxSub = 'zxx is 25 years old'
    console.log(zxxAge) // zxx is 8 years old
    // console.log(zxxSub) // zxxSub is not defined
    function zxxFnSub () {
        var zxxSub = 'zxx is 18 years old'
        console.log(zxxSub) // zxx is 18 years old 
        console.log(zxxAge) // zxxSub is not defined
    }
    zxxFnSub()
}
zxxFn()
```