reuse-pending-promise

Reuses a promise by reference until it's settled (resolved or rejected). It helps ensure only one fetch is made at a time--preventing multiple simultaneous fetches for the same data.

It decorates the fn (a thennable) with a cache of pending promises. The cached promise is returned until it has been fulfilled.

reusePendingPromise(fn[, options])

Install

npm install reuse-pending-promise

Example usage:

import { reusePendingPromise } = from 'reuse-pending-promise'

let callCount = 0
// A promise-returning function
const MyFn = () => {
  callCount++
  return new Promise(resolve => {
    // Wait 10 seconds before resolving
    setTimeout(() => {
      resolve(callCount)
    }, 10 * 1000)
  })
}

// Wrap the promise returning function
const reusedMyFn = reusePendingPromise(MyFn)

Promise
  // Call the reused function 3 times in parallel
  .all([reusedMyFn(), reusedMyFn(), reusedMyFn()])
  .then(() => console.log(`callCount: ${callCount}`))
  // now that the first promise has resolved...
  .then(reusedMyFn)
  .then(() => console.log(`callCount: ${callCount}`))

// 10 seconds later, the initial three calls resolve but the `myFn` is only invoked once:
// // callCount: 1

// 20 seconds later, the last call will resolve, having invoked `myFn` a second time.
// // callCount: 2

Using getCacheKey option for variations

The getCacheKey option can be used to cache variations based on the fn's arguments, similar to the resolver argument in lodash.memoize. By default the first argument is used as the cache key.

Here's an example where promises are re-used only if the lang and country arguments match:

const fetchData = (lang, country) => fetch(`http://example.com/${country}/${lang}`)
const getCacheKey = (lang, country) => `${lang}${country}`

const reusedFetchData = reusePendingPromise(fetchData, { getCacheKey })
// promise1 and promise2 will share the same promise returned by fetch,
// since they share a cache key.
const promise1 = reusedFetchData('en', 'canada')
const promise2 = reusedFetchData('en', 'canada')
// promise1 === promise2

// promise3 will be a different cache key, thus will be a new promise for
// a second second fetch call.
const promise3 = reusedFetchData('fr', 'canada')
// promise3 !== promise1

Additional examples are in the unit test

Contributing

Fork and create a PR.

License

This project is MIT licensed.