JavaScript boy and “lovely thoughtleader” 
he/him 
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Local variable refinements are unsafe in TS and Flow. Both are bad, TS is the worst. Seems obvious in hindsight but neither handle local refinement invalidation correctly. I'm not speaking abstractly, this has caused bugs in my code before.
Flow at least invalidates refinements when calling local functions that mutate variables, but does not follow any indirection https://flow.org/try/#0PQKgBAAgZgNg9gdzAZwC4CcCWBjVYTABQhMApngG4CGMAXGAHYCuAtgEanpgA+KGmDAOZgAvGADk4gNzEoTBrkxwGYKsmSZBDAJIMAJpnSlFygBQBKMAG9CYVes0MAcqw7oLMgL6z5JlWo0tF3ZOC2tbMGoYUTAAJi8fBVQlFRYqATCbO0woMFNUAE8AB1I4XKjRETFxZhD0cUssuzBUAAt0REZSJABRdA73cSc4RldOZFUYeARSPQaZO28I4GAwbVy2zAmENTAAAwDHYLc9ltbSFQAxabAEOCYYPTBsOH7jVBgCsAEozD0qVCkM5AoxQASkFgXVARQ5aXQGIx+DzEOwrMAAFVaWxQrXujzAUHSMAANN9UOIJlRRnUIqYovQ0FghOYvEA …
TS won't ever invalidate local refinements in this case, even when it can easily statically tell that a function call will https://www.typescriptlang.org/play/#code/DYUwLgBAbghsBcEB2BXAtgIxAJwgHwgGcxsBLJAcwgF4IByOgbgChmAzFJAYzFIHskEGIUKkKSAHLos2ABQBKCAG9mEaHBoQATCwC+rDt14CIaGOQXLVEUmwiywATwAOIPndjAa1WnVSYcOkUVNTUwAAtsPgB3ZBBYgFFsKLk6CT5kaRxCIWBgGJAAEyCWNX1rAHoKiAARATpIJBAioSRHGyRC0mwQHn5BcJwQa2FRcSkAuXkWSuqAFXDSHMJwvhRgQog2c2AAGhswOhyYTMnrLgFiIhJEYjJKTU89IA …
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