Promise.defer + flow

defer.js

// @flow

export type Deferred<T> = {
  promise: Promise<T>,
  resolve: (value: T) => void,
  reject: (error: Error) => void,
}

export function defer(): Deferred<any> {
  const deferred = {}
  deferred.promise = new Promise((resolve, reject) => {
    deferred.resolve = resolve
    deferred.reject = reject
  })
  return deferred
}