javascript - 我对 javascript 中变量的范围感到困惑

Question

我正在练习 Javascript，所以我写道:

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

结果令我惊讶:

/*the console result*/
window
2
2
NaN

在我看来，this 指向f();。 b 是 f(); 的私有(private)变量。 this.b++ 和 b++ 对同一变量进行操作。

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

请解释为什么我没有得到预期的结果。

Answer 1

让我们分解一下这里发生的事情:

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);