为什么引用未声明的变量会引发引用异常，而引用未声明的属性却不会呢? [英] Why does referencing undeclared variables throw a reference exception but referencing undeclared properties doesn't?
问题描述
Mozilla说变量是全局对象的属性.如果对象具有未定义的属性，则尝试访问它不会创建引用异常-它只是返回未定义该属性.
Mozilla says that variables are properties of the global object. If an object has a property that isn't defined, then trying to access it does not create a reference exception - it simply returns that the property is not defined.
如果存在这样的全局对象-那么为什么访问不存在的属性(即变量)会导致引用错误?这两种情况之间到底有什么区别?
If there is such a global object - then why does accessing its properties (ie: variables) that do not exist create reference errors? What is precisely the difference between these two scenarios?
示例:
console.log(x) //x is not declared -> reference error
var x = new Object();
console.log(x.property); //console.log: undefined
推荐答案
tl; dr:访问变量的方式会带来所有不同，而不是变化的方式存储.
tl;dr: The way the variable is accessed makes all the difference, not how it is stored.
首先了解一些背景:
基本上有两种方式存储"变量.
There are basically two ways how variables are "stored".
在
In a declarative environment, variables are stored in an internal data structure that is not accessible from user code.
在 object 环境中，变量存储为用户代码可访问对象的属性.全局环境恰好是一个对象环境(比它复杂一些，但让它保持简单).因此，全局变量是全局对象的属性.
In an object environment, variables are stored as properties of a user code accessible object. The global environment happens to be an object environment (it's a bit more complicated than that but lets keep it simple). Therefore global variables are properties of the global object.
那么为什么访问不存在的全局变量会引发错误?因为与变量的存储方式 无关，所以仅访问.
So why does accessing a non-existing global variable throw an error? Because it is not relevant how the variables are stored, only how the are accessed.
foo 和 window.foo 是访问全局变量的两种不同方式.
foo and window.foo are simply two different ways of accessing a global variable.
用于评估变量( foo )的语言规则明确指出，如果变量不存在，则抛出引用错误(同样，无论如何存储).*
The language rules for evaluating a variable (foo) explicitly say that if the variable doesn't exist, throw a reference error (again, no matter how it is stored).*
用于评估属性访问权限的语言规则(<> window.foo )表示，如果该属性不存在，则应返回 undefined .
The language rules for evaluating property access (window.foo) say that if the property doesn't exist, undefined should be returned.
如果您考虑一下，从一致性的角度来看，这更有意义.无论变量是存储在声明性环境还是对象环境中，访问 foo 都应具有相同的结果.
And if you think about, this makes much more sense from a consistency perspective. Accessing foo should have the same result, no matter whether the variable is stored in a declarative environment or an object environment.
*:更精确地说:这是 GetValue 函数，该函数将引发错误.解析变量时，几乎在任何地方都会调用 GetValue (例外情况是 typeof 和分组运算符(...)).
*: To be more precise: It's the GetValue function that causes the error to be thrown. GetValue is called almost everywhere when resolving a variable (exceptions are typeof and the grouping operator (...)).
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