Memoize an async function, caching the resolved value and deduplicate concurrent calls

memoizePromise.ts

/** Memoizes an async function, caching the resolved value and deduplicating concurrent calls */
export const memoizePromise = <T>(fn: () => Promise<T>): (() => Promise<T>) => {
  let cached: T
  let hasCached = false
  let pending: Promise<T> | null = null

  return () => {
    if (hasCached) {
      return Promise.resolve(cached)
    }
    if (!pending) {
      pending = fn().then(value => {
        cached = value
        hasCached = true
        pending = null
        return value
      })
    }
    return pending
  }
}