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Microsoft announces Python formulas in Excel... which have to get sent to the cloud
(techcommunity.microsoft.com)
This is a most excellent place for technology news and articles.
I think what they let you do is write mini add-ons using the API and support JS in an in-browser editor. Then you have to enable the add-ons. I guess equivalently you can do the same things with an IDE and any language, but it felt like they "officially" supported JS.