creates a cycle to intentionally ensure only `a` can run `b`

a.mjs

import {ran} from 'b';
let ready;
// this function can be saved to a variable, but blows up if `b` has not run yet
export function onlyAvailableIfBHasRun() {
  ready;
  ran;
  // ...
};

b.mjs

import {onlyAvailableIfBHasRun} from 'a';
// ready to init
export const ran = null;
onlyAvailableIfBHasRun();
// ... do side effects ...