为什么外部作用域变量没有正确地绑定到内部变量? [英] Why doesn't the outer scope variable get properly bound to the inner variable?

问题描述

(function(){
  var x = 23;
  return function(){
    var x = x;
    return x;
  }
}())();

为什么它返回undefined而不是23?

Why does it return undefined instead of 23?

不应该var x = x;部分足够明确，因为右侧隐式指的是外部范围中的x?

Shouldn't the var x = x; part be sufficiently unambiguous because the right hand side implicitly refers to the x in the outer scope?

推荐答案

语句 var x = x; 从外部范围看不到变量 x .范围内的变量 x 在赋值之前已经存在，并且从外部范围隐藏该变量.

The statement var x = x; doesn't see the variable x from the outer scope. The variable x inside the scope already exists before the assignment, and shadows the variable from the outer scope.

作用域中的所有变量都是在作用域代码执行(提升)之前创建的，因此与您具有的作用相同:

All variables in the scope are created before the code in the scope executes (hoisted), so it's the same as if you had:

(function(){
  var x;
  x = 23;
  return function(){
    var x;
    x = x;
    return x;
  }
}())();

---

实际上，在变量中声明变量的位置并不重要.您可以在代码中最后声明它们(尽管这会有些混乱)，并且代码仍然可以正常工作:

---

It actually doesn't matter where in the scope you declare the variables. You can declare them last in the code (although that would be a bit confusing), and the code still works the same:

(function(){
  x = 23;
  return function(){
    x = x;
    return x;
    var x;
  }
  var x;
}())();

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