将值与'undefined'进行比较的最佳方法是什么？
var a;
(a == undefined)
(a === undefined)
((typeof a) == "undefined")
((typeof a) === "undefined")
我们应该使用哪一个？
具有讽刺意味的是， undefined 可以在JavaScript中重新定义，而不是那些心智正常的人会这样做，例如：
Ironically, undefined can be redefined in JavaScript, not that anyone in their right mind would do that, for example:
undefined = "LOL!";
此时所有未来的等式检查 undefined 会产生意想不到的结果！
at which point all future equality checks against undefined will yeild unexpected results!
至于 == 和之间的差异== = （相等运算符），==将尝试将值从一种类型强制转换为另一种类型，用英语表示 0 ==0即使类型不同（Number vs String），它也会评估为true - 开发人员倾向于避免这种类型的松散相等，因为它可能导致代码中的调试错误。
As for the difference between == and === (the equality operators), == will attempt to coerce values from one type to another, in English that means that 0 == "0" will evaluate to true even though the types differ (Number vs String) - developers tend to avoid this type of loose equality as it can lead to difficult to debug errors in your code.
因此最安全：
"undefined" === typeof a
检查未定义时：）