javascript - 为什么 'this' 没有采用正确的范围

Question

好吧，我知道 Javascript 中有几千个关于 this 范围的奇怪线索(这让人怀疑这种语言是否设计得很好) - 但我仍然无法解释“this”一:

//works
function Cat() { 
 this.theCatName = "Mistigri"; 
 function meow() {alert(this.theCatName + " meow") }; 
 this.meow = meow }
}
var MyCat = new Cat()
MyCat.meow() 

//works
function Cat() { 
this.theCatName = "Mistigri"; 
function meow() {alert(this.theCatName + " meow") }; 
this.meow = function() { alert(this.theCatName + " meow") }
}
var MyCat = new Cat()
MyCat.meow() 

//works
function Cat() { 
this.theCatName = "Mistigri"; 
function meow() {alert(this.theCatName + " meow") }; 
Cat.prototype.meow = function() { alert(this.theCatName + " meow") }
}
var MyCat = new Cat()
MyCat.meow() 

//doesn't work
function Cat() { 
this.theCatName = "Mistigri"; 
function meow() {alert(this.theCatName + " meow") }; 
this.meow = function() { meow() } }
}
var MyCat = new Cat()
MyCat.meow() 

//doesn't work
function Cat() { 
this.theCatName = "Mistigri"; 
function meow() {alert(this.theCatName + " meow") }; 
Cat.prototype.meow = function() { meow() } }
}
var MyCat = new Cat()
MyCat.meow() 

现在我的理解是，在后两种情况下 Cat.prototype.meow 或 this.meow 是恰好调用 meow() 的匿名函数，即Cat() 的内部函数 - 但 this 的上下文清楚地引用了函数内部的 cat - 它会发生什么？

这是一个半规范的答案: See How to access the correct this / context inside a callback? 但它仅说明了“this”的实际上下文:

this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. It is not affected by lexical scope, like other variables.

Answer 1

当您调用对象的方法时，只有当您将其作为对象的成员调用时，它才有效，如果您获取对该函数的引用或将其作为常规函数调用，则上下文不是该对象。只有调用函数的方式决定了上下文，上下文不会被继承，因此如果您从方法调用函数，那么它只是常规函数调用。

示例:

function Dog() {

  function bark() {}
  this.bark = bark;

  this.barkThrice() {
    bark(); // function call; context = window
    this.bark(); // method call; context = the object
    var b = this.bark;
    b(); // function call; context = window
  }

}

要将函数作为对象的方法调用，您需要使用 call method (或bind或apply)设置调用的上下文:

function Cat() { 
  this.theCatName = "Mistigri"; 
  function meow() { alert(this.theCatName + " meow"); } 
  this.meow = function() { meow.call(this); };
}
var MyCat = new Cat();
MyCat.meow();

演示:http://jsfiddle.net/k6W9K/