Deferred Promise
The DeferredPromise class is a Promise-compatible abstraction that defers resolving/rejecting promises to another closure. This class is primarily useful when one part of your system establishes as promise but another part of your system fulfills it.
This class is conceptually inspired by the
createDeferredPromise()internal utility in Node.js. Unlike the Node.js implementation, however,DeferredProimseextends a nativePromise, allowing the consumer to handle deferred promises like regular promises (no.promiseinstance nesting).
Getting started
npm install @open-draft/deferred-promise
Documentation
Class: DeferredPromise
new DefferedPromise()
Creates a new instance of a deferred promise.
import { DeferredPromise } from '@open-draft/deferred-promise'
const promise = new DeferredPromise()
Unlike the regular Promise, a deferred promise does not accept the callback function. Instead, you should use .resolve() and .reject() to resolve and reject the promise respectively.
A deferred promise is fully compatible with the native Promise, which means you can pass it to the consumers that await a regular Promise as well.
deferredPromise.state
<"pending" | "resolved" | "rejected">Default:"pending"
const promise = new DeferredPromise()
console.log(promise.state) // "pending"
promise.resolve()
console.log(promise.state) // "resolved"
deferredPromise.result
Returns the value that has resolved the promise. If no value has been provided to the .resolve() call, undefined is returned instead.
const promise = new DeferredPromise()
promise.resolve('John')
console.log(promise.result) // "John"
deferredPromise.rejectionReason
Returns the reason that has rejected the promise. If no reason has been provided to the .reject() call, undefined is returned instead.
const promise = new DeferredPromise()
promise.reject(new Error('Internal Server Error'))
console.log(promise.rejectionReason) // Error
deferredPromise.resolve()
Resolves the deferred promise with a given value.
function startServer() {
const serverReady = new DeferredPromise()
new http.Server().listen(() => {
// Resolve the deferred promise with the server address
// once the server is ready.
serverReady.resolve('http://localhost:8080')
})
// Return the deferred promise to the consumer.
return serverReady
}
startServer().then((address) => {
console.log('Server is running at "%s"', address)
})
deferredPromise.reject()
Rejects the deferred promise with a given reason.
function createBroadcast() {
const runtimePromise = new DeferredPromise()
receiver.on('error', (error) => {
// Reject the deferred promise in response
// to the incoming "error" event.
runtimePromise.reject(error)
})
// This deferred promise will be pending forever
// unless the broadcast channel receives the
// "error" event that rejects it.
return runtimePromise
}
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