This is a well explored area. Wikipedia article is short but has some good references [0]. Languages like Haskell, OCaml, F#, and Elm use global type inference

This is a well explored area. Wikipedia article is short but has some good references [0]. Languages like Haskell, OCaml, F#, and Elm use global type inference. Languages like Scala and TypeScript use local type inference. TypeScript in particular uses flowed type inference (there might be a more formal name for this). For example: let a = document.querySelector('#a') // typeof a is Element | null if (a == null) throw 'not found' // typeof a is now Element [0] https://en.wikipedia.org/wiki/Type_inference https://en.wikipedia.org/wiki/Type_inference