[adgasf]

Conversely, it's also one of the biggest pain-points when throwing together a quick prototype.

[navaati]

Nah, you can totally use things like holes or undefined (in haskell), unimplemented!() in rust, etc.

EDIT: and, as neighbour says, any in typescript ! But typescript, with its progressive typing, is in a league of its own.

[worldsayshi]

Then here's where typescript shines. To be able to move fluidly between no static checks of JavaScript and strong-ish type safety of typescript without too much work.

Then again, it's not always without hiccups.

[jefflombardjr]

That is where I think Flow really shines. Seems to be less hiccups, but that is just my anecdotal experience.