ES4 attempted to add type support. (It's the ES version time forgot, mostly known only for forks it spawned like Adobe's ActionScript.)
Typescript and Flow both resemble, to different degrees, the type support that ES4 attempted (pascal-style `: typename` annotations being common to all of them).
Presumably the reasons browsers were not interested in ES4 remain the case today.
If that changes, then Typescript/Flow are useful playgrounds to explore what the standard could be. Many ES proposals now require "prollyfills" exploring that feature in the space of existing browsers as proof-of-concept, to help explore possible complications. Typescript and Flow do good jobs of "prollyfilling" type support, and I would expect that if a new attempt to standardize types in ES were to occur, they would both be looked to for advice/guidance.
Typescript and Flow both resemble, to different degrees, the type support that ES4 attempted (pascal-style `: typename` annotations being common to all of them).
Presumably the reasons browsers were not interested in ES4 remain the case today.
If that changes, then Typescript/Flow are useful playgrounds to explore what the standard could be. Many ES proposals now require "prollyfills" exploring that feature in the space of existing browsers as proof-of-concept, to help explore possible complications. Typescript and Flow do good jobs of "prollyfilling" type support, and I would expect that if a new attempt to standardize types in ES were to occur, they would both be looked to for advice/guidance.