When comparing GHCJS vs Flow, the Slant community recommends GHCJS for most people. In the question“What are the best solutions to "The JavaScript Problem"?” GHCJS is ranked 8th while Flow is ranked 32nd. The most important reason people chose GHCJS is:
With a Haskell backend, GHCJS enables code sharing. In combination with the power of Haskell as a language, this enables an extremely tight integration of the client side with the server side, where all the communications take place in a type-safe manner and even transparently if desired.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Full Stack Code Sharing
With a Haskell backend, GHCJS enables code sharing. In combination with the power of Haskell as a language, this enables an extremely tight integration of the client side with the server side, where all the communications take place in a type-safe manner and even transparently if desired.
Pro All of Haskell, with the same tools you're used to
No need to learn new syntax or semantics, and no need to install and learn a bunch of new tools - it's just GHC.
Pro Mature language and community
Pro Terse
Haskell is a very terse language, particularly due to its type inference. This means there's nothing to distract from the intent of the code, making it very readable. This is in sharp contrast to languages like Java, where skimming code requires learning which details can be ignored. Haskell's terseness also lends itself to very clear inline examples in textbooks, and makes it a pleasure to read through code even on a cellphone screen.
Pro Quick Feedback
It's often said that, in Haskell, if it compiles, it works. This short feedback loop can speed up learning process, by making it clear exactly when and where mistakes are made.

Pro Checks to see if you check for Nulls
Because getting those exceptions is just not fun and very pervasive.
Pro Versioned type definitions
Pro There is support in many code editors via the extension
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.
Pro Babel extension for strip of type annotations
Thanks to the Babel extension for the output, there is minimally modified code that is understandable to the author.
Pro Statical analysing of JavaScript code
Statical analysing of JavaScript code without pre-making any changes to it. But supported annotation types by extending the syntax of the language.
Cons
Con Large runtime
GHCJS supports the entire Haskell runtime, the Javascript it outputs tends to be quite large. This is in contrast to options such as Fay, which save some overhead by not supporting some features such as multi-threading.
Con Weak base type definitions even for popular JavaScript libraries
For example, there are definitions for Gulp, React.
