Fay vs Flow

Fay produces smaller output than pure Haskell compilers such as GHCJS; It does not need to include the whole Haskell runtime, as it drops support for features such as multi-threading, giving it fewer dependencies.

As with UHC, the FFI to Javascript works with printf-style format:

max = ffi "Math.round(%1,%2)"

This can simplify code needed to make calls to methods on objects, in contrast to e.g., Purescript's FFI, which requires that methods be wrapped in Javascript. Similarly to UHC, Fay also supports the use of %*, for javascript functions with arbitrary numbers of parameters, such as concat, though they must expose an explicit number of parameters to Fay.

Fay is available on Cabal, as are Fay packages, so getting up and running is as simple as typing 'cabal install'. Happstack, Snap, and Yesod packages are available on Hackage, as are bindings for JQuery and Backbone.

Since Fay is a subset of Haskell - Lazy, statically typed, and pure by default. There's no new syntax to learn, and no surprises when it comes to the semantics of your code. This extends into function names as well - Fay programmers can use familiar functions such as putStrLn to output to the console, rather than Javascript-specific versions.

Because getting those exceptions is just not fun and very pervasive.

For example, there is good support through the extension in Visual Studio Code, which is a good editor for TypeScript, which is a competitor to Flow.

Thanks to the Babel extension for the output, there is minimally modified code that is understandable to the author.

Statical analysing of JavaScript code without pre-making any changes to it. But supported annotation types by extending the syntax of the language.