Flow also supports "discriminated unions" [0], and did so before TypeScript (Flow's announcement in July 2015: https://flow.org/blog/2015/07/03/Disjoint-Unions/, TypeScript's announcement in August 2016: https://devblogs.microsoft.com/typescript/announcing-typescr...), though it doesn't yet support destructuring the "kind" field as was just added to TypeScript.

[0] https://flow.org/try/#0C4TwDgpgBAggxsAlgewHZQLwCgq6gHygG8oBr...

As somebody who never worked with Flow but loves TS, what does Flow offer that the latter can’t do?

Flow feels pretty dead. The VSCode extension is broken in major ways and hasn’t gotten a major update in years. The only mainstream editor that fully supports it is IntelliJ.