我感到困惑，在JavaScript中varible

Question

我練的Javascript的範圍，所以我寫了：

function f(){ 
     console.log(this);  
     var b=2; 
     console.log(b); 
     this.b++;  
     console.log(b); 
     b++; 
} 
f(); 
console.log(b); 

，結果讓我大吃一驚：

/*the console result*/ 
window 
2 
2 
NaN 

在我看來，this點f(); 。 b是f();的私有變量。 this.b++和b++對相同的變量進行操作。

/*the right anwser in my mind*/ 
f 
2 
4 
TypeError 

請解釋爲什麼我沒有得到預期的結果。

Answer 1

要得到4你必須這樣做 this.b = this.b ++; 或 b = b ++;也將起作用。

由於b未在該範圍內定義，因此您得到Nan作爲最後一個結果。有關更多詳細信息，請參閱JavaScript中的變量提升。

Answer 2

讓我們打破這裏發生了什麼：

function f(){ 
    /* 
    Since you called this function directly, "this" is pointing 
    to the window object of the browser because it is the global scope 
    */ 
    console.log(this); // window 
    var b=2; // Assigning 2 to a local variable "b" 
    console.log(b); // 2 - because you are logging your local scoped variable 

    /* 
     This is really saying window.b++ and since b isn't typically 
     defined on the window object it is essentially saying: 
     window.b = undefined + 1 
    */ 
    this.b++; 
    console.log(b); // 2 - because you are logging your local scoped variable that hasn't changed 
    b++; // Increment your local scoped variable 
} 

f(); // Calling the function you just defined in the global scope 

/* 
    This is actually logging window.b which is considered to be 
    NaN (not a number) because you incremented "undefined" and 
    since undefined is not a number it can't be incremented 
*/ 
console.log(b); 

Answer 3

你有b變量設置爲局部變量在f功能（使用var b），所以你不能使用window變量訪問。

如果設置（在外部範圍的var b）的b可變全球，你將能夠使用this（如window）做到這一點：

var b 
 
function f(){ 
 
     // console.log(this); 
 
     b=2; 
 
     console.log(b); 
 
     this.b++;  
 
     console.log(b); 
 
     b++; } 
 
f(); 
 
console.log(b);

我已經禁用了第一個console.log(this)，因爲這樣的片段給出了很多輸出