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;
这篇关于在javascript中检查undefined--我应该使用typeof吗？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！