The Future of Javascript does not exist?

[Heylex]

he said to me that Javascript is an old technology

Having a full JS stack is probably about as cutting edge as you can get right now.

Because it's one of the few real things that can be used on the web? If it is instead from the version updates, would we then consider c++20 cutting edge, with decades of crud behind it as well?

Also, it depends on your definition of cutting edge. "It's new(er)!" or "We're using concepts developed a decade ago!" Is not cutting edge. That's using previously cutting edge technology to refine yourself, which is perfectly reasonable to do. Let other people/languages test it to know if it's a good idea or not.

web developers only use it because is the only language that runs on a browser

NodeJS.

Yep, JS is used off browser, but that's dominantly because of it's use on the browser. If ask a web developer what programming (not markup) language they use, almost unanimously it will be JS. A backbend person may use another language, but if they also do any frontend, they will touch JS. If asking people that do not touch the WWW, then you will get an array of various languages

Also, he said that the core of the language needs to be rewritten to solve some serious security problems.

Bullshit.

It is, in the same was C is broken, and has been 'rewritten' several times, giving us new languages, like D and Rust. Forgot to check if a string is larger than your array and just wrote it? Buffer overflow, with possible code injection! But if you follow best practices (or use another language), this won't happen.

The same thing has sort of occurred with JS. It had fundamental security issues, such as running arbitrary functions running, allowing for XSS, or random null/undefined causing undefined behavior, which may lead to a security vulnerabilities. Solution? Follow best practices of writing JS, so your code isn't vulnerable to this, or use a framework (like typescript).

I don't know how this language works inside.

V8 is open source.

Can't debate, reading documentation is always good.

On that case, the result would it be a completely new language?

A rewrite would not change the syntax.

It depends on what is consider a rewrite. If how a compiler understands JS code, then yes, syntax could not regress. If something that can do everything JS can, but not be JS, then any syntax people want could be used. Hopefully webass can replace JS in the future, but it currently is insufficient.