JavaScript 预解析机制与四种典型场景解析

标准预解析与变量提升

var num=10;
  fun();
function fun(){
  console.log(num);//undefined
var num=20; }

var num;
      function fun(){
        var num;
        console.log(num);//undefined
        num=20;        
      }
      num=10;
      fun();

局部变量遮蔽与函数内提升

var num=10;
       function dn(){
         console.log(num);//undefined
         var num=20;
         console.log(num);//20
       }
       dn();      
       //相当于以下代码
       var num;
       function dn(){
         var num;
         console.log(num);
         num =20;
         console.log(num );
       }
       num=10;
       dn();

var a =18;
       ss();
       function ss(){
         var b=9;
         console.log(a);//undefined
         console.log(b);//9
         var a='123';
       }       
       //相当于以下代码       
       var a;
       function ss(){
         var b;
         var a;
         b=9;
         
         console.log(a);
         console.log(b);
         a='123';
       }
       a=18;
       ss();

连续赋值与全局变量访问限制

aa();
       console.log(c1);//9
       console.log(b1);//9
       console.log(a1);//报错       
       function aa(){
         var a1=b1=c1=9;
         console.log(a1);//9
         console.log(b1);//9
         console.log(c1);//9
       }       
       //相当于
       function aa(){
         var a1=b1=c1=9;//相当于 var a=9;b=9,c=9
         var a1;
         a1=b1=c1=9;
         
         console.log(a1);
         console.log(b1);
         console.log(c1)
       }
       aa();
       console.log(c1);
       console.log(b1);
       console.log(a1);