Ask for Promise

Decouple a Promise from its resolve / reject and let any code complete it.

A standard Promise is born inside an executor function, and only that function can resolve or reject it. ask-for-promise flips that around — it returns a Promise together with its resolve and reject handles as plain object properties, so you can call them from anywhere: a callback, an event handler, a timer, another promise, or a sync function.

Why

The moment an async task finishes is rarely the same place where the task was started. ask-for-promise is a small DX layer for those cases:

• Race conditions — share one done between N parallel callbacks to emulate Promise.race.
• Long chains — declare each step as a named askForPromise() and wire them up without nesting .then.
• Mixed sync / async — a function that's sometimes sync can still call task.done().
• Legacy callback APIs — pass task.done straight to a Node-style callback.
• Timeouts — race any promise against a TTL with .timeout(ttl, fallback).

Features

• Decoupled done / cancel — call them from anywhere
• onComplete sugar for promise.then, with an optional reject handler
• .timeout(ttl, fallback) — race against a timer
• askForPromise(list) — one AskObject for N promises, with .each for iteration
• askForPromise.sequence(list, ...args) — run steps in order, threading results as args
• askForPromise.all(list, ...args) — run steps in parallel
• TypeScript types included
• Zero runtime dependencies

Installation

npm install ask-for-promise

// ES modules
import askForPromise from 'ask-for-promise'

// CommonJS
const askForPromise = require('ask-for-promise')

Quick start

import askForPromise from 'ask-for-promise'

const task = askForPromise()

// Call `done` from anywhere — even a callback API:
setTimeout(() => task.done('done'), 1000)

// `onComplete` is sugar for `task.promise.then`
task.onComplete((result) => {
  console.log(result) // → 'done'
})

task.promise is a real Promise. task.done is its resolve. task.cancel is its reject. Anything you can do with a normal Promise works here too.

API

askForPromise() → AskObject

Creates a single promise and returns its AskObject.

const task = askForPromise()

askForPromise(list) → AskObject

Creates one promise per item in list and returns a single AskObject that controls them all. task.promise resolves to an array of values in input order when every sub-promise resolves.

const task = askForPromise(['a', 'b', 'c'])

askForPromise.sequence(list, ...args) → AskObject

Runs each function in list one after the other. The result of each function is appended to args and passed to the next — useful for piping values through async steps.

const steps = [loadUser, loadPosts, renderPage]
askForPromise.sequence(steps, userId).onComplete(([user, posts, html]) => {
  // ...
})

askForPromise.all(list, ...args) → AskObject

Runs each entry in list in parallel. Entries can be functions (called with ...args) or already-running promises; the result array is in declaration order.

askForPromise.all([fetchA, fetchB, fetchC], token).onComplete(([a, b, c]) => {
  // ...
})

AskObject

| Property | Description | | --- | --- | | promise | The underlying Promise. In list mode this is Promise.all(promises). | | promises | AskObject[] in list mode, null in single mode. Use task.promises[i].done(value) to resolve a single item. | | done(value?) | Resolves the promise(s) with value. In list mode, resolves every sub-promise with the same value. | | cancel(reason?) | Rejects the promise(s) with reason. In list mode, rejects every sub-promise with the same reason. | | onComplete(resolveFn, rejectFn?) | Sugar for promise.then. Pass a second function as the reject handler. | | each(cbFn, ...args) | Calls cbFn({ value, done, cancel, timeout }, index, ...args) per item — index is the position in the input list. | | timeout(ttl, fallback) | Arms a ttl-millisecond timer. On expiry, the task settles with fallback: onComplete is rewired to return the fallback, and the underlying task.promise is settled with the same value (so await task.promise and task.onComplete(...) agree). In list mode, each still-pending sub-promise is replaced with fallback while sub-promises that already settled keep their real value. Returns the same AskObject so you can keep chaining. |

Examples

Simple promise

const task = askForPromise()

function asyncWork() {
  // ... do something
  task.done('task complete')
}

task.onComplete((result) => {
  console.log(result) // → 'task complete'
})

Promise.race, the manual way

A single shared done produces a race — the first callback to call it wins, the rest are no-ops.

const task = askForPromise()

slowAsync((err, r) => task.done('slow'))
fastAsync((err, r) => task.done('fast'))

task.onComplete((winner) => {
  console.log(winner) // → 'fast'
})

Long chain without nesting

Declare each step as a named variable, then wire them up:

const prepareFolders  = askForPromise()
const writeFiles      = askForPromise()
const updateInterface = askForPromise()

myFS.makeFolders(folders, () => prepareFolders.done())

prepareFolders
  .onComplete(() => {
    myFS.writeFiles(files, () => writeFiles.done())
    return writeFiles.promise
  })
  .then(() => {
    updateInterface()          // sync part of the work
    updateInterface.done()
    return updateInterface.promise
  })
  .then(() => {
    console.log('DONE')
  })

Promise with timeout

timeout returns the same AskObject, so you can keep chaining.

const task = askForPromise().timeout(2000, 'expire')

setTimeout(() => task.done('success'), 5000)

task.onComplete((result) => {
  if (result === 'expire') console.log('timed out')
  else                     console.log(result)
})

In list mode, the timer applies to the whole group:

const task = askForPromise([job1, job2, job3]).timeout(1000, 'expire')

List of promises with .each

task.each(cb) walks the list and hands you a per-item { value, done, cancel, timeout } plus the item's index:

const files = ['info.txt', 'general.txt', 'about.txt']
const task  = askForPromise(files)

task.each(({ value, done, cancel }, index) => {
  console.log(`writing ${value} (#${index})`)
  fs.writeFile(value, 'dummy text', (err) => {
    if (err) cancel(err)
    else      done()
  })
})

task.onComplete(() => console.log('DONE'))

task.promise here is equivalent to Promise.all(task.promises.map(p => p.promise)). To resolve a single item rather than all of them, call task.promises[i].done(value).

TypeScript

Type definitions are bundled — no extra install.

import askForPromise, { AskObject } from 'ask-for-promise'

const task: AskObject = askForPromise()

License

MIT — see LICENSE.

Credits

Created by Peter Naydenov.

Changelog

Changelog.md