Didn’t both of them start around the same time? React released close a year later but it reportedly had been running internally at Facebook for a couple years by then.

It’s also a relatively modest extension of decades of prior art so I think it might be more accurate to treat it as a formalization of that practice rather than implying it was foundational.

Perhaps it's a case of convergent evolution, since React similarly has a strong functional influence (immutable tree nodes in the VDOM).

Definitely - it’s always tricky tracing the history of these things since these ideas are coming out of the same larger pool of experience and convention, and people are understandably prone to identifying an idea with the first place they encountered it, lacking visibility into other pockets of the primordial soup.

Looking at the language used to describe it on https://guide.elm-lang.org/architecture/ I’m struck by the resemblance to the message-passing systems which were common in the 90s and 80s due to the influence Smalltalk had in the 70s.

Seems like both of them are inspired by Functional Reactive Programing, so yeah, hard to say Elm was foundational

Is that the case? I thought Bonér's Reactive Manifesto came out around the same time and was based on early Scala work.