console.log(a)  //functiona(){   var a = 'a';}
// a = 'test';

function a(){
    console.log(b)
    a = 'a'; // 暗示全局變量
    function b (){

    }
}
a(); 
console.log(a)

輸出:

function a(){
    console.log(b)
    a = 'a'; 
    function b (){
    }
}
function b(){

} 
a

原因：

• js執(zhí)行前會(huì)進(jìn)行預(yù)編譯
  • 全局預(yù)編譯GO(Global Object)
    • 創(chuàng)建GO對(duì)象
    • 給全局變量賦值 undefined GO{a: undefined}
    • 將全局的函數(shù)申明的函數(shù)名作為key,value為函數(shù)整體賦值到GO對(duì)象中
      • GO{a:function a(){
        ? console.log(b)
        ? a = 'a'; // 暗示全局變量
        ? function b (){}
        }}
  • 函數(shù)預(yù)編譯AO (Activation Object)
    • 創(chuàng)建AO對(duì)象
    • 將函數(shù)內(nèi)的形參和變量聲明存儲(chǔ)到AO對(duì)象中，值為undefined
    • 將實(shí)參和形參統(tǒng)一
    • 將函數(shù)內(nèi)的函數(shù)申明的名稱作為AO對(duì)象的key，函數(shù)的整體內(nèi)容為value 存儲(chǔ)到AO對(duì)象
      • AO{b:function b(){}}

說(shuō)明

• GO=window對(duì)象，GO即操作window，a為全局變量,console.log(a) = console.log(window.a)