Redux Middleware: Cacheable Promises

promise.js

// Utility to check if value is a Promise
function isPromise(value) {
    if (value !== null && typeof value === 'object') {
        return value && typeof value.then === 'function'
    }

    return false
}

// Cache
let cache = {}

export default function promiseMiddleware({ dispatch }) {
    return (next) => (action) => {
        // If it's not a promise, skip
        if (!isPromise(action.payload)) {
            return next(action)
        }

        // Check the cache
        if (cache.hasOwnProperty(action.type)) {
            // In the cache, return Promise in progress
            return cache[action.type]
        } else {
            // Not in cache, trigger pending action
            next({
                type: `${action.type}_PENDING`
            })

            // Handle Promise resolution
            const handleResolution = (error = false, payload = null) => {
                // Remove from cache
                delete cache[action.type]

                return dispatch({
                    type: `${action.type}_${error ? 'REJECTED' : 'FULFILLED'}`,
                    payload,
                    error
                })
            }

            // Add Promise to cache and return it
            return cache[action.type] = action.payload.then(handleResolution.bind(null, false), handleResolution.bind(null, true))
        }
    }
}