Javascript预解析

变量提升 函数提升

原代码

var num = 10
fun();

function fun() {
    console.log(num)
    var num = 20
}

预解析

var num //变量提升

//函数提升
function fun() {
    var num   //变量提升到作用域的最前面
    console.log(num)//此时num未赋值
    num = 20
}

num = 10
fun();

控制台输出 undefined

原代码

f1();

console.log(b);
console.log(c);
console.log(a);
function f1() {
    var a = b = c = 9
    console.log(a);
    console.log(b);
    console.log(c);
}

预解析

function f1() {
    //var a = 9
    // b = 9
    // c = 9
    //不是集体声明，b和c是全局变量赋值
    var a = b = c = 9
    console.log(a); //9
    console.log(b); //9
    console.log(c); //9
}

f1();
console.log(b); //9
console.log(c); //9
console.log(a); //报错 外部不能调用局部变量