Use of if(typeof value !== 'undefined')?

[Fdawgs]

Noticed this in a few places in this repo:

https://github.com/fastify/fastify-helmet/blob/b0aacaee932e1629a6233c97424d739735b3293c/index.js#L25C31-L25C31

Never seen this before, does this have a performance benefit over the following?:

• if(value)
• if(value !== undefined)

[Uzlopak]

More like differenciating between null and undefined.

[gurgunday]

Now, this is a little nuanced

In nearly all cases, we should definitely prefer value === undefined to typeof value === “undefined” as checking against undefined is simply faster

However, there are times where you want to check if a top level variable even exists in the first place

For example, to check if variable b exists at the most outer scope, you can't use if(b !== undefined) {} because it will throw a ReferenceError when b isn't defined

For that, if(typeof b !== 'undefined') {} must be used. The typeof operator will silence the error and just return 'undefined'

So, it'd be great if you could find a way to create a script that mass updates the repos and we reviewed them one by one to see if the typeof use is indeed redundant 😁

[Fdawgs]

@gurgunday will make PRs and point them back to this issue.