js预解析

console.log(num)

Uncaught ReferenceError: num is not defined

console.log(num)
var num = 10

    fn();//正常执行
    function fn(){
        console.log('hello')
    }
    fn();//正常执行

fun();//Uncaught TypeError: fun is not a function
    var fun = function(){
        console.log('hello')
    }
    fun();//正常执行

// console.log(num)
    // var num = 10
    var num;
    console.log(num)//undefined
    num=10;

fn();
    console.log(c)
    console.log(b)
    console.log(a)
    function fn(){
        var a = b = c = 9;
        console.log(a)
        console.log(b)
        console.log(c)
    }

9
9
9
9
9
Uncaught ReferenceError: a is not defined

/*分析如下*/
    //1.函数提升
    function fn(){
        /*1.1变量提升*/
        var a;
        a = b = c = 9;
        // 相当于var a = 9 b = 9 c = 9
        //b和c没有var 变成了全局变量
        console.log(a)
        console.log(b)
        console.log(c)
    }
    //2.代码执行
    fn();
    console.log(c)//9
    console.log(b)//9
    console.log(a)//全局没有a变量 所有显示未定义