I don't know what to make of the fact that reactivity doesn't seem to be discussed in the docs or the tutorial. I am a fan of ObservableHQ's reactive JavaScript notebooks, in which blocks of code get rerun when their inputs change. This is reactivity in the same sense as spreadsheets are reactive, and it's pretty neat. One nice feature of ObservableHQ is that you can use free variables when writing code blocks, and they will get their values (reactively) from the environment. That is very powerful and it seems to me that plugging a scripting language into a spreadsheet should work that way as well.