empty-promise

Constructs an empty promise (pun intended) that can be resolved or rejected from the outside.

Instead of wrapping your promise in around your logic code, you may pass an empty promise through your code and resolve it later.

Can be quite useful for writing easier to read asynchronous tests. Also solves difficult problems in complex scenarios involving multiple paradigms of streams, events, callbacks, etc.

This style of coding shows potential for significantly simplifying promise based designs, but also shows potential to turn your code into a confusing mess. Approach it cautiously until we can hash out a few best practices.

Not to be confused with Promise.resolve() and Promise.reject() from the native Promise API. These native methods only create a fully resolved or fully rejected promise on the fly. (The simplest possible empty promise usage in the first example is functionally equivalent to using Promise.resolve().)

Install

npm install --save empty-promise

Requires Node v6+

Basic Usage

The bare minimum: Here we create an empty promise, then resolve it with "hello promise" and pass the results to console.log().

import emptyPromise from 'empty-promise'

emptyPromise()
  .resolve('hello promise')
  .then(console.log) // logs 'hello promise'

// Alternatively you may use Nodejs style require
const emptyPromise = require('empty-promise')

It's asynchronous: Just like any other promise, you can pass it around and add then() or catch() calls before it resolves.

const wait = emptyPromise()

wait
  .then((val) => {
    console.log(`val`) // 'some value'
  })
  .catch((err) => {
    console.err(err) // does not call because we did not reject
  })

setTimeout(() => {
  wait.resolve('some value')
}, 1000)

Works with ES6 async/await: Here we repeat the previous example inside an async IIFE.

const wait = emptyPromise()

(async ()=> {
  let value

  try { value = await wait }
  catch (err) {
    console.error(err)
  }

  console.log('${value}') // 'some value'
})()

setTimeout(() => {
  wait.resolve('some value')
}, 1000)

Only resolves once: Important to reiterate that an empty promise is basically still just a promise. Like any other promise, once it resolves, it will not resolve again.

const promise = emptyPromise()

[42, 2, 17].forEach(async number => {
  console.log(number)  // 42, 2, 17
  const resolvedNumber = await promise.resolve(number)
  console.log(resolvedNumber) // 42, 42, 42
})

New feature- done status: Now we can call done() method on an empty promise to find out it's resolved status. Returns true if resolved or rejected, and false if still pending. This can be used for optimizing or cancelling expensive actions that might wastefully attempt to resolve the promise a second time.

const promise = emptyPromise()

[42, 2, 17].map(number => {

  if (promise.done())
    return promise

  return promise.resolve(number)

}).forEach(console.log) // 42, 42, 42

See Empty Promise Playground for more usage.