New Observable web editor over the new Notebook 2.0 programming model https://new.observablehq.com/ Its support more languages now (including Typescript and vanilla Javascript) with backwards compatible cells for the old ObservableJS syntax (a.k.a. Observable Javascript).

Wasn't Observable JavaScript used in order to do automatic reactivity? Did they get around that somehow to enable it in TS/JS

yes they get automatic reactivity in the TS/JS case. Mike Bostock said they did not know how to do it without ObserveableJS originally but have since figure it out. It was not clear how to map reactive cells to plain Javascript. In OJS a cell outputs one value. In Notebook 2.0 code, {let a = 5, let b = 10} is a single cell that has two values (a and b). So old ObservableJS a cell had multiple inputs and one output update per tick. Observable 2.0 is multiple inputs and outputs per tick which is more expressive and can map to normal Javascript blocks easier.