Simple observable store

store.mjs

/**
 * Simple observable value store.
 * @template {any} T
 * @param {T | Promise<T>} initial
 */
export function Store(initial) {
  /** @type {Set<(newValue: T) => void>} */
  const subscribers = new Set();
  const store = { value: Promise.resolve(initial) };
  return {
    /** @param {T} value */
    set(value) {
      store.value = Promise.resolve(value);
      subscribers.forEach((callback) => {
        store.value.then(callback);
      });
    },
    get: () => store.value,
    /** @param {(newValue: T) => void} callback */
    subscribe(callback) {
      subscribers.add(callback);
      store.value.then(callback);
      return () => void subscribers.delete(callback);
    },
  };
}

usage.mjs

import { Store } from './store.mjs';

const store = Store(
  fetch('https://api.github.com/orgs/withastro/repos')
    .then(res => res.json())
);

store.subscribe(json => console.log(json));