Fwiw, that comment doesn't appear to say undefined now:

If you want to make sure you understand something, try to explain it out loud to a rubber duck.

(Not kidding.)

Edit: Y'all, I'm a programmer. I know the origins. But it's useful in learning anything - not just debugging!

https://www.reddit.com/r/AskReddit/comments/lum9el/comment/gp8b4kv

Would need to see some more examples to try and derive a pattern, and I don't really understand what I'm looking at in your screenshot or what the connection is to YouTube, but based on that sample of one I'm wondering if the undefined somehow crept in when the original comment was updated?

Edit: Ok, I worked out what the screenshot is showing and oh my god do I hate those videos with a passion. But I digress. My updated guess is that they're using an API to access the comments and automatically create the videos, and that the API can't cope with edited comments and outputs "undefined" instead. Or they're scraping the website instead of using an API, but in that case it's harder to imagine why it can't successfully scrape an edited comment.

At any rate, it seems like it's probably reasonable to assume this is a coding error somewhere along the way, probably related to edited comments, and not something comment/post authors are doing themselves.

There are many things that you can do with undefined and null. I recommend reading about them on MDN here (for null) and here (for undefined).

One difference I had to work with recently was the behavior when using JSON.stringify:

let fooUndefined = { foo: undefined };
let fooNull = { foo: null };

console.log(JSON.stringify(fooUndefined));
console.log(JSON.stringify(fooNull));

The output:

{}
{"foo":null}

As you said, it does come down to the difference between not being defined (undefined) and having it defined as nothing (null). There are many times this comes up!

Another difference is with typeof:

> typeof undefined
'undefined'
> typeof null
'object'

If you're curious, I once wrote a blog post on typeof null. It's a sneaky bug from the original implementation of JavaScript!

They are shadowbanned?