Cleaner attributeFilter/attributeOldValue example (MutationObserver)

[mrienstra]

Description

Simplify callback, in the MutationObserver example illustrating the usage of attributeFilter & attributeOldValue.

Motivation

I thought the old version had a typo, in userStatusChanged(mutation.target.username, mutation.target.status);, mutation.target.username seems like it should've been mutation.oldValue, though after taking a closer look, this was probably intentional.

While fixing this "typo", I decided the old callback was unnecessary complex -- e.g. the outer switch wasn't doing anything (aside from serving as a future placeholder perhaps) -- so I made it a bit more "to the point". Also seems a tiny bit unusual to have an example that calls functions not provided (userStatusChanged & usernameChanged), this new one is a little easier to modify and begin playing with.

Related issues and pull requests

Old version added in #9737:
https://github.com/mdn/content/pull/9737/files#diff-fc20b9f4adbc6723e9fe01425cea6512399d9a9bd69451d25f8fd6f0b8f92788R144-R170

[github-actions]

Preview URLs

• /en-US/docs/Web/API/MutationObserver/observe

[Josh-Cena]

Thanks! However, as mentioned in the example explanation, this example is meant to display a real-world relevant example of why you may want to use this API. In general, it's much easier to adapt a real-world example to a runnable example (either stub out the missing function implementation, or replace the function body with a simple log, as you have done), but it's hard to imagine how an example can be real-world relevant by looking at a contrived example like the new one. For example, it's easy to image that userStatusChanged and usernameChanged will each send a request to the server saying "the status of user X has changed to Y", and "the username of user X has changed to Y".