That’s not dataflow analysis. It looks like a cast similar to something like in C# ie: if (foo is string as foo) where the “as foo” scoped variable is implicit. It’s basically implicitly redefining the variable with limited scope. Type guards are a slick feature but it is just an expression match and some syntactic sugar. Even user defined type guards in typescript have to be explicitly declared as such.

This would be more an example of CFA.

  const a = “string”;
  if (typeof v === a)