Zarith is an OCaml library that implements common operations over arbitrary-precision integers and rationals. It is implemented via a C api that primarily calls out to Gnu Multiple Precision Arithmetic Library. Because of that C API, Zarith could not be compiled via Js_of_ocaml until now.
Zarith Stubs JS is a reimplementation of the native C functions in JavaScript. It makes extensive use of peterolson/BigInteger.js as a shim for browser BigInt functionality and also for the implementations of many numerical algorithms.