In theory Flow has a stronger inference engine than TypeScript, which means it's more suited to incremental adoption.
TypeScript is based on an AST; which means it's inferences are limited. For example it requires you to type the parameters to a function. If the return type is derived from untyped parameters then TypeScript falls back to typing the return type as 'any'.
Flow maps the flow (hence the name) of types throughout an application, which means that it can derive the signature of a function based on the types that are passed into it. That should mean that it's a little easier to add to an existing project.
TypeScript can do type refinements based on the flow (e.g. a null check refines a type with null to one without) but can't do what you just showed. Are there any tools that can emit the inferred types to source code? I just now got a vision of using flow style type inference to gradually augment a project with either flow or TS types with very little work.
Flow has an API that can expose its inferences. There are Flow to TypeScript conversion tools that might bake the types into definitions.
Ironically, we’re considering switching to TypeScript and in my team the lack of inference is seen as an advantage. It forces people to consider their types as part of the design of their interfaces.
