cached-request.js

let cache = null;

export function myCachedRequest() {
  if (cache) {
    return Promise.resolve(cache);
  }

  return fetch('potato.com/foo').then(x => {
    cache = x;
    return x
  })
}

export function getCachedValue() {
  return cache;
}

usage.js

import { myCachedRequest, getCachedValue } from './cached-request.js';

// if cached it will be resolved immediately
// otherwise it will make a request
myCachedRequest().then(value => {
  // ...
})

// syncronous but if the value is not cached yet returns null
const value = getCachedValue()