ProxyPromise

A ProxyPromise is a Promise that can be manually settled and inspected. It decouples the creation of the ProxyPromise object from the conditions that will resolve or reject it.

For example, a function dispatching tasks to background workers can create a ProxyPromise and return it immediately. A worker can later grab the ProxyPromise and settle it, while the original caller is waiting.

npm install proxy-promise

The package includes Typescript definitions.

Usage

The ProxyPromise constructor takes no parameters:

const promise = new ProxyPromise()

You can invoke .resolve(value) or .reject(error) to settle the ProxyPromise at any later time:

const p1 = new ProxyPromise()
p1.resolve(10)

const p2 = new ProxyPromise()
p2.reject(new Error("ouch"))

Like a regular Promise, a ProxyPromise will ignore any calls to resolve or reject after the first one.

Calling then or catch on the ProxyPromise will return a regular Promise.

new ProxyPromise()
  .then(result => console.log(result)) // now a regular `Promise`

new ProxyPromise()
  .catch(error => console.error(error)) // now also a regular `Promise`

Only the original ProxyPromise object has the resolve and reject methods. You can prevent other actors from settling the promise by calling then, giving them a regular Promise.

Inspection

You can inspect a ProxyPromise by looking at its state property. The state is set to 'pending' at creation, then to 'resolved' or 'rejected'.

• When state is 'resolved', the ProxyPromise will have a value property.

• When state is 'rejected', the ProxyPromise will have an error property.

Extras

The resolve and reject methods of a ProxyPromise are already bound, so they can be passed as arguments without using .bind().

The ProxyPromise also comes with a callback property, a classic-style function(error, value) that will invoke reject(error) or resolve(value), depending on whether error == null.

const promise = new ProxyPromise()

promise.then(value => console.log(value))

fs.readFile('README.md', promise.callback)