在javascript中检查undefined--我应该使用typeof吗？ [英] checking for undefined in javascript-- should I use typeof or not?

问题描述

我对如何最好地检查javascript中是否未定义变量感到困惑。我一直这样做：

I'm a bit confused about how best to check if a variable is undefined or not in javascript. I've been doing it like this:

myVar === undefined;

但在所有情况下使用typeof更好吗？

But is it better in all cases to use typeof instead?

typeof myVar === undefined;

那么使用 undefined vs undefined，我也见过？

And what about the use of undefined vs "undefined", which I've also seen?

推荐答案

这个是最好的检查方式 - 完全万无一失：

This is the best way to check -- totally foolproof:

typeof myVar === "undefined"

这没关系，但如果有人无意中覆盖全局 undefined ，它可能会失败值：

This is OK, but it could fail if someone unhelpfully overwrote the global undefined value:

myVar === undefined;

必须说ECMAScript 5指定 undefined 是只读的，所以上述内容在任何符合的浏览器中都是安全的。

It has to be said that ECMAScript 5 specifies that undefined is read-only, so the above will always be safe in any browser that conforms.

这将永远不会有效，因为它最终会比较undefined=== undefined （不同类型）：

This will never work because it ends up comparing "undefined" === undefined (different types):

typeof myVar === undefined;

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