As of RN 0.60, Hermes JS engine can be used on Android. To check whether that engine is enabled, RN provides the global.HermesInternal variable which could/should go in Global (name pending further discussion) along with __DEV__.

However, adding proper bindings is a bit non-sensical. What you have is a variable which has the value undefined when Hermes is disabled and {} when enabled (really!).

So the bindings can be either

module Hermes = {
	type t;

	[@bs.val] [@bs.scope "global"]
	external status: t = "HermesInternal";

	external disabled : unit => t = "#undefined"
	[@bs.obj] external enabled: unit => t = "";
};

using the established way to create an empty object, as used on Js.Dict.empty(), etc. In use, one might have

Hermes.status == Hermes.enabled()

such a function call may be avoided as below

module Hermes = {
	type t;

	[@bs.val]
	external status: t = "HermesInternal";

	external disabled : t = "#undefined"
	[@bs.val]
	external enabled: ([@bs.as {json|{}|json}] _) => t =
  "Object.assign";
};

which would be used as

Hermes.status == Hermes.enabled

The latter introduces a bit of noise due to Object.assign on the JS side.

IMO, it's better to simply add a function (not zero-cost, per se but that cost would be incurred at the call site anyway) as below

let (isHermesEnabled: bool) = [%bs.raw {| global.HermesInternal != null |}];

(it really should be compared to undefined, but perhaps following documentation may be more future proof).

Any opinions?