makeCancelable.js

// @flow

export type PromiseHandler = {
	promise: Promise<*>,
	cancel: () => void,
}

export default function makeCancelable(promise: Promise<*>): PromiseHandler {
	let hasCanceled = false

	const wrappedPromise = new Promise((resolve, reject) => {
		promise.then(
			(val) => hasCanceled ? reject({ isCanceled: true }) : resolve(val),
			(error) => hasCanceled ? reject({ isCanceled: true }) : reject(error)
		)
	})

	return {
		promise: wrappedPromise,
		cancel() {
			hasCanceled = true
		},
	}
}