[GeZe]

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...