javascript - typeof！==“undefined”vs.！= null(typeof !== “undefined” vs. != null)

Question

I often see JavaScript code which checks for undefined parameters etc. this way:

(我经常看到JavaScript代码检查未定义的参数等，这样：)

if (typeof input !== "undefined") {
    // do stuff
}

This seems kind of wasteful, since it involves both a type lookup and a string comparison, not to mention its verbosity.

(这似乎有点浪费，因为它涉及类型查找和字符串比较，更不用说它的冗长。)

It's needed because 'undefined' could be renamed, though.

(这是必要的，因为'undefined'可以重命名。)

My question is: How is that code any better than this approach:

(我的问题是：代码如何比这种方法更好：)

if (null != input) {
    // do stuff
}

As far as I know, you can't redefine null, so it's not going to break unexpectedly.

(据我所知，你不能重新定义null，所以它不会意外地破坏。)

And, because of the type-coercion of the != operator, this checks for both undefined and null... which is often exactly what you want (eg for optional function parameters).

(并且，由于！=运算符的类型强制，这将检查undefined和null ...这通常正是您想要的（例如，对于可选的函数参数）。)

Yet this form does not seem widespread, and it even causes JSLint to yell at you for using the evil != operator.

(然而，这种形式似乎并不普遍，它甚至会导致JSLint对你使用邪恶！=运算符大喊大叫。)

Why is this considered bad style?

(为什么这被认为是不好的风格？)

  ask by Derek Thurn translate from so

Answer 1

typeof is safer as it allows the identifier to never have been declared before:

(typeof更安全，因为它允许标识符从未在之前声明：)

if(typeof neverDeclared === "undefined") // no errors

if(neverDeclared === null) // throws ReferenceError: neverDeclared is not defined