js-promise-middleware

A middleware wrapper for promises that simplifies side effect management.

The problem

You have a reactive application and are too lazy to learn how to use observables?

Example

Import it

import PromiseMiddleware from 'js-promise-middleware'
// unless you are building a basic app, you should be using your own middleware, not this
// check out the code used to build this simple middleware in src/middleware
import { cache, dedupe } from 'js-promise-middleware/middleware'

Declare your fetcher

// create a simple fetchUser function that returns a promise that resolves with the user
const fetchUser = id => fetch(`/users/${id}`).then(res => res.json())

// wrap that fetcher with promise middleware
const UserFetcher = new PromiseMiddleware(fetchUser)

// tell the cache how to get the entity's id based on the arguments the fetch request is given
const idGetter = id => id

// cache requests using a normalized state tree
cache(idGetter)(UserFetcher)

// dedupe requests to the same user
dedupe(idGetter)(UserFetcher)

// because cache and dedupe are composable, you can also compose middlewares
compose(
  cache,
  dedupe
)(UserFetcher)

export default UserFetcher

Use it

import UserFetcher from 'fetchers/User'
// every time the UserFetcher successfully resolves, fire this callback
// args will be an array of the arguments provided when we call "request" and res will be whatever the promise
// originally given to the PromiseMiddleware resolved with
// returns a callback to unsubscribe the middleware
const unsubscribe = UserFetcher.onSuccess(onUserFetchSuccess)

function onUserFetchSuccess({ res, args }) {
  console.log(`Got the user with the id ${idGetter(...args)}!`, res)
}

UserFetcher.request(1)
UserFetcher.request(1) // does not call the server because we are deduping requests to the same user
UserFetcher.request(2)

// ...sometime later
UserFetcher.request(2) // does not call the server because we applied a cache to this fetcher
unsubscribe() // when we no longer care about request events, we can unsubscribe to avoid memory leaks
//

Publishing

To publish this as an npm package, use yarn publish-package rather than the usual yarn publish. This runs a custom npm script that builds the js-promise-middleware, moves the package.json file to the es directory and publishes from there. It allows consumers of this npm package to use your dependency by writing import someModule from 'js-promise-middleware/someModule' rather than import someModule from 'js-promise-middleware/es/someModule'