javascript - ReferenceError 和全局对象

Question

在浏览器的 JavaScript 中，window 是全局对象，这意味着在全局范围内定义的每个变量都是 window 的子对象。那么为什么我会得到这个结果:

console.log(window.foo); // No error, logs "undefined".
console.log(foo);        // Uncaught ReferenceError: foo is not defined.

Fiddle

这两行应该是一样的，不是吗？

Answer 1

因为对于 window.foo，您正在明确地寻找 window 对象的 foo 属性，而在后一个选项中并非如此。在后一个选项中，如果未定义 foo，作为开发人员，您应该能够知道它未定义并获得明确的错误警告，而不是解释器将其设置为 undefined 本身(如第一种情况)会导致意外结果。

Reference Error :

Represents an error when a non-existent variable is referenced. A ReferenceError is thrown when trying to dereference a variable that has not been declared.

查看这篇文章了解更多信息:

• Understanding JavaScript’s ‘undefined’

引用上面的文章:

A Reference is considered unresolvable if its base value is undefined. Therefore a property reference is unresolvable if the value before the dot is undefined. The following example would throw a ReferenceError but it doesn’t because TypeError gets there first. This is because the base value of a property is subject to CheckObjectCoercible (ECMA 5 9.10 via 11.2.1) which throws a TypeError when trying to convert Undefined type to an Object.

示例:

var foo;
foo.bar; //TypeError (base value, foo, is undefined)
bar.baz; //ReferenceError (bar is unersolvable)
undefined.foo; //TypeError (base value is undefined)

根据定义，既不是属性也不是变量的引用是不可解析的，并且会抛出 ReferenceError，因此:

foo; //ReferenceError