BuckleScript (OCaml) https://github.com/bloomberg/bucklescript is the only one that produces reasonable output that doesn't require large runtime libraries all the time.
Koka https://github.com/koka-lang/koka is a very interesting research language that focuses on explicitly specifying effects in types. It even deconstructs Haskell's notion of purity into "throws exceptions" + "may never terminate". Unfortunately, the code generation (which BTW exists for both JS and C#) is uh… not very good.
But honestly, I haven't felt the need to use a functional language in frontend development. JavaScript (ES6) is good enough, and it's actually supported in browsers, so you can develop without any compiling, and only use Babel to make production builds. If the project is not small, use TypeScript to add some type safety.
Currently I'm writing in Purescript but I'm constantly checking BuckleScript and I'm liking direction of it. What is OCaml polymorphism story? Does it have analogue of typeclasses? I'm currently using Purescript for writing build steps for webpack and I benefit from Haskell ideas like lenses, refolds and free monads. It would be great if I could transfer those tools. Maybe you could point me to some literature about transforming recursive data structures (ASTs) in OCaml? Maybe thera are some interesting tools and techniques?
Koka https://github.com/koka-lang/koka is a very interesting research language that focuses on explicitly specifying effects in types. It even deconstructs Haskell's notion of purity into "throws exceptions" + "may never terminate". Unfortunately, the code generation (which BTW exists for both JS and C#) is uh… not very good.
But honestly, I haven't felt the need to use a functional language in frontend development. JavaScript (ES6) is good enough, and it's actually supported in browsers, so you can develop without any compiling, and only use Babel to make production builds. If the project is not small, use TypeScript to add some type safety.