event-emitting-promise

An extended Promise object implementing Node's EventEmitter methods

Reasoning

From time to time, I found myself in the need to combine the comfortable program flow that Promises offer (especially with async/await) with the verbose usefulness of events.

My usual use case is the creation of CLI versions from functions I've wrote. Using this package, that can be done pretty easily.

Install

# With yarn
yarn add event-emitting-promise

# With npm
npm install --save event-emitting-promise

Usage

Let's say we want to create a programmatic and a CLI version of a function called myFunc().

Consider this the my-func.js:

const EventEmittingPromise = require('event-emitting-promise')

// Asynchronous partial tasks
const { task1, task2, task3 } = require('./somewhere')

module.exports = function myFunc () {
  // The EventEmittingPromise constructor callback takes an
  // additional argument called "emit", which is the
  // EventEmitter.prototype.emit method.
  return new EventEmittingPromise(async (resolve, reject, emit) => {
    try {
      const result1 = await task1()
      emit('partial', 'task1', result1)

      const result2 = await task2()
      emit('partial', 'task2', result2)

      const result3 = await task3()
      emit('partial', 'task3', result3)

      resolve({
        task1: result1,
        task2: result2,
        task3: result3
      })
    } catch (err) {
      reject(err)
    }
  })
}

And this the my-func-cli.js:

const myFunc = require('./my-func')

module.exports = function myFuncCLI () {
  return myFunc()
    // The created Promise provides all of the EventEmitter methods
    // (except emit) and allows chaining them.
    .on('partial', (name, result) => {
      console.log(
        `Partial task ${name} has finished with result:`,
        result
      )
    })
    .then(() => {
      console.log('Done.')
    })
    .catch(err => {
      console.error('An error occurred:', err)
    })  
}

That way, places calling myFunc() can easily be informed about the progress of the function call.