是的，这是检查两者的好方法。语言规范陈述（关于 == 用于比较不同类型的值）：

Yes, that's a nice way to check for both. The language specification states (regarding == for comparing values of different types):

2）如果x为null且y为undefined，返回true。

2) If x is null and y is undefined, return true.

3）如果x未定义且y为null，则返回true。

3) If x is undefined and y is null, return true.

此处 x 和 y 是比较的条款 x == y 。当您比较 x == null 时，只有 x 时才会为真undefined ，或 null 本身。

Here x and y are the terms of a comparison x == y. When you're comparing x == null, it will only be true if x is undefined, or null itself.

为了清楚起见，当我们在这里说未定义时，我们指的是值 undefined ，而不是未定义的变量（无论何时使用它们都会产生ReferenceError，除了 typeof ）。

Just to be clear, when we say "undefined" here we mean the value undefined, not variables that are not defined (those produce a ReferenceError whenever they're used, except with typeof).

关于WWDCD，我'引用Ian： Crockford会建议JSLint建议（显然），因为==对他来说是邪恶的。这意味着使用 === ，绝不会 == 。所以这对他来说不是问题。

And regarding WWDCD, I'll quote Ian: Crockford would suggest what JSLint suggests (obviously) because == is voodoo to him. That means "use ===, never ==". So this would be a non-question for him.