使用'var undefined'来测试undefined是否有任何问题(Is there anything wrong with using 'var undefined' to test for undefined)

这个测试对于undefined有什么问题吗？

var undefined; if(x == undefined){ //do something }

或这个：

function undefined(x){ return typeof x == 'undefined'; } if(undefined(x)){ //do something }

jsLint不会抛出存储的单词错误，但代码似乎仍然工作...

Is there anything wrong with this test for undefined?

var undefined; if(x == undefined){ //do something }

or this:

function undefined(x){ return typeof x == 'undefined'; } if(undefined(x)){ //do something }

jsLint doesn't throws a reserverd word error, but the code still seems to work...

最满意答案

不要重新定义undefined 。

其他程序员期望undefined总是undefined ，而不是函数的功能。

人们经常使用typeof运算符来确保在用于测试undefined变量时引用错误不会被抛出。

如果有人向你这样做，你可以使用...

undefined = void 0;

...将其还原。

Don't redefine undefined.

Other programmers expect undefined to always be undefined, not a function for function's sake.

People often use typeof operator to ensure a reference error is not thrown when used to test for variables that are undefined.

If anyone ever does this to you, you can use...

undefined = void 0;

... to revert it back.