Because Javascript in a browser is not normally multi-threaded, and ocamljs makes no attempt to implement threads under the hood, it is very convenient to use the monadic cooperative threads of Lwt. Lwt under ocamljs is also useful with orpc.
The core of Lwt can be compiled under ocamljs (but of course the parts that depend on the Unix library may not be used). To build it, unpack the Lwt source (matching to the installed version) in a directory adjacent to the ocamljs source, then configure, build, and install as usual. You can then use the Findlib lwt package as usual with ocamljs.
In addition there are a few functions in Lwt_dom (Findlib package lwt-dom) for timers etc. See the Ocamldoc.