A promise that can be resolved on demand.

promiseOnDemand.ts

import Emitter from 'emitter20'

/** Returns a [promise, resolve] pair. The promise is resolved when resolve(value) is called. */
const promiseOnDemand = <T>(): [Promise<T>, (value: T) => void] => {
  const emitter = new Emitter()
  const promise = new Promise<T>((resolve, reject) => {
    emitter.on('resolve', resolve)
  })

  /** Triggers the emitter to resolve the promise. */
  const resolve = (value: T) => emitter.trigger('resolve', value)

  return [promise, resolve]
}