Looking into the history of reactive updates, we find that it started with simple javascript commands helped kickstart most of it.

https://en.wikipedia.org/wiki/Ajax_(programming)

The idea of reactivity started in the 1990's in production.

When Gmail was released this technology is what made a website behave like a desktop app (plus the huge amount of storage)

If we were to look into today's equivalent of doing this, it might be surprising what exists in the standard libraries.