Node.js — Callback and Promise Support in your Node.js Modules

Node.js developers are very familiar with the callback pattern. During the last years, a common practice has taken place within the community: error-first callbacks. That was quite sufficient for the last years. Now that promises and generator functions are around the corner, it’s time to start thinking outside the box.

Return callbacks as well as promises from your functions to be both, backward-compatible and future proof!

Node.js Series Overview

Why?

You’re absolutely right and we should clarify the necessity on why you should return a callback AND a promise from a function?

The first and foremost reason: provide backward compatibility. If you’ve published a library to NPM, there are developers using your module and it would be pretty shoddy to break it and stop callback support.

The second reason: be future proof. Promises are coming, no matter how loud you speak against them. That’s why you should enable promise support in your module for those developers that already build their apps using promises.

Example Function With Callback and Promise Support

The following code snippet illustrates a function that supports callbacks as well as promises. The packageInfo function reads the contents of your package.json file, returns with an error or the actual file content as callback and promise.

Please note, that you need to make sure your callback is initialized appropriately so that the code doesn’t trap into issues when using promises where you don’t pass a callback function and the actual call of your callback would result in a callback is not a function error.

'use strict'

const fs = require('fs')  
const path = require('path')  
const packagePath = path.resolve(__dirname, '..', 'package.json')

function packageInfo (callback) {  
  return new Promise((resolve, reject) => {
    Fs.readFile(packagePath, (err, data) => {
      if (err) {
        // if no callback available, reject the promise
        // else, return callback using "error-first-pattern"
        return callback ? callback(err) : reject(err)
      }

      return callback ? callback(null, data) : resolve(data)
    })
  })
}

module.exports = packageInfo  

Node.js v4 ships with native promises support. That means you don’t need a 3rd party library to provide the additional functionality. If you want to support an earlier version of Node.js, please add a library that implements promises.

Node.js 8.0 ships with "Util.promisify"

In situations where you're only interested in promisifying a callback-style function, benefit from the newly added "promisify" utility. It's available in Node.js v8 and returns a promise-based version of a function that is actually in callback-style.

The following snippet illustrates the promisifying process of Fs.readFile:

'use strict'

const Fs = require('fs')  
const Path = require('path')  
const Util = require('util')  
const packagePath = path.resolve(__dirname, '..', 'package.json')

promisifiedPackageFileRead function() {  
  const readFilePromise = Util.promisify(Fs.readFile)
  return readFilePromise(packagePath)
}

module.exports = promisifiedPackageFileRead  

The return value of Util.promisify is a function that returns promises instead of the callback. In this case, readFilePromise returns a promise with the file's content instead of an error-first callback.

Outlook

Lift your modules and applications to the next level by supporting promises besides callbacks. Developers out there will thank you for that move, existing ones and those using your module in the future.

Did we miss an important piece or do you want further explanations, please let us know in the comments below or shoot us a message on Twitter: @futurestud_io.

Enjoy coding & make it rock!

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