JS引擎的工作方式：先解析代碼，獲取所有變量的聲明。

• 變量提升

    var a = 1;
    var b = 2;
    var c = 3;

等價(jià)于

    var a;
    var b;
    var c;
    a = 1;
    b = 2;
    c = 3;

所以

    console.log(a);//undefined 不會(huì)報(bào)錯(cuò)
    console.log(c);// c is not defined 報(bào)錯(cuò)
    var a = 1;
    var b = 2;

等加于：

    var a;
    var b;
    console.log(a);//undefined 不會(huì)報(bào)錯(cuò)
    console.log(c);// c is not defined 報(bào)錯(cuò)
    a = 1;
    b = 2;

• 函數(shù)聲明提升
• 當(dāng)函數(shù)通過函數(shù)聲明進(jìn)行定義時(shí)，會(huì)有聲明提升

   printNum();//有效，控制臺(tái)打印了1
   function printNum(){
       console.log('1');
   }

• 但是，通過其他方式定義的函數(shù)沒有聲明提升

1.構(gòu)造函數(shù)

   printNum();//報(bào)錯(cuò) printNum is not a function(...)
   var printNum = new Function("console.log('1');");

2.函數(shù)表達(dá)式

   printNum();//報(bào)錯(cuò) printNum is not a function(...)
   var printNum = function(){
       console.log(1);
   };