I find https://ellx.io/ very interesting, after chatting with its creator on HN a while ago in another next-gen spreadsheets discussion.
At first glance, it's just a web-native spreadsheet where formulas and macros are both JavaScript instead of Excel formulas and VBA. However, it's clever in doing some introspection and automatic functional-reactive lifting in cases where inputs to the formulas are external time-varying inputs. You write something that looks like regular JavaScript, but if one or more of your inputs is actually a time-varying function, then the result is a time-varying function that looks like a regular JavaScript variable but is automatically updated whenever its dependencies change. So you get something like the automatic updating semantics of Excel expressions, which is very different from the semantics of regular JavaScript variables.
I seem to also remember some clever introspection abilities on those time-varying functions, but it's been a while. In any case, it's obvious the creator has thought long and hard about how web-native spreadsheets should behave.
It's also interesting in that you can either explicitly name the cell's result using var_name = expression or else just have a bare expression, in which case an implicit variable name is created, similar to Excel's $A$4 naming of cell results.
It does have limited calc, but I tend to think of it as more of an afterthought. The scripting is “very incomplete” in the words of the author, consider e.g. https://github.com/aardappel/treesheets/issues/37
https://strlen.com/treesheets/
And this software is at least 9 years old :)