Prefer arrow functions and callbacks (#1210)

src/actions/register.ts

@@ -2,7 +2,7 @@ import { Action } from "./types";
 
 export let actions: readonly Action[] = [];
 
-export function register(action: Action): Action {
+export const register = (action: Action): Action => {
   actions = actions.concat(action);
   return action;
-}
+};