Deduplicates concurrent async calls by sharing a single in-flight Promise. Useful for preventing duplicate network requests or expensive computations.

create-shared-promise.js

export function createSharedPromise(fn) {
  let promise = null;

  return (...args) => {
    if (!promise) {
      promise = Promise.resolve().then(() => fn(...args)).finally(() => {
        promise = null;
      });
    }
    return promise;
  };
}

Use case:

const fetchUser = createSharedPromise(() => fetch('/api/user').then(r => r.json()));

// Even if called 5 times simultaneously → only 1 request
await Promise.all([fetchUser(), fetchUser(), fetchUser()]);