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

  1. String Replace All Appearances
  2. Remove All Whitespace From a String in JavaScript
  3. Generate a Random ID or String in Node.js or JavaScript
  4. Remove Extra Spaces From a String in JavaScript or Node.js
  5. Remove Numbers From a String in JavaScript or Node.js
  6. Get the Part Before a Character in a String in JavaScript or Node.js
  7. Get the Part After a Character in a String in JavaScript or Node.js
  8. How to Check if a Value is a String in JavaScript or Node.js
  9. Check If a String Includes All Strings in JavaScript/Node.js/TypeScript
  10. Check if a Value is a String in JavaScript and Node.js
  11. Limit and Truncate a String to a Given Length in JavaScript and Node.js
  12. Split a String into a List of Characters in JavaScript and Node.js
  13. How to Generage a UUID in Node.js
  14. Reverse a String in JavaScript or Node.js
  15. Split a String into a List of Lines in JavaScript or Node.js
  16. Split a String into a List of Words in JavaScript or Node.js
  17. Detect if a String is in camelCase Format in Javascript or Node.js
  18. Check If a String Is in Lowercase in JavaScript or Node.js
  19. Check If a String is in Uppercase in JavaScript or Node.js
  20. Get the Part After First Occurrence in a String in JavaScript or Node.js
  21. Get the Part Before First Occurrence in a String in JavaScript or Node.js
  22. Get the Part Before Last Occurrence in a String in JavaScript or Node.js
  23. Get the Part After Last Occurrence in a String in JavaScript or Node.js
  24. How to Count Words in a File
  25. How to Shuffle the Characters of a String in JavaScript or Node.js
  26. Append Characters or Words to a String in JavaScript or Node.js (Coming soon)
  27. Check if a String is Empty in JavaScript or Node.js (Coming soon)
  28. Ensure a String Ends with a Given Character in JavaScript or Node.js (Coming soon)
  29. Left-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  30. Right-Trim Characters Off a String in JavaScript or Node.js (Coming soon)
  31. Lowercase the First Character of a String in JavaScript or Node.js (Coming soon)
  32. Uppercase the First Character of a String in JavaScript or Node.js (Coming soon)
  33. Prepend Characters or Words to a String in JavaScript or Node.js (Coming soon)
  1. Get Number of Seconds Since Epoch in JavaScript
  2. Get Tomorrow’s Date in JavaScript
  3. Increase a Date in JavaScript by One Week
  4. Add Seconds to a Date in Node.js and JavaScript
  5. Add Month(s) to a Date in JavaScript or Node.js
  6. Add Week(s) to a Date in JavaScript or Node.js
  7. Get the Current Year in JavaScript or Node.js
  8. How to Get a UNIX Timestamp in JavaScript or Node.js
  9. How to Convert a UNIX Timestamp to a Date in JavaScript or Node.js
  10. Add Days to a Date in JavaScript or Node.js
  11. Get Yesterday's Date in JavaScript or Node.js
  12. Add Minutes to a Date in JavaScript or Node.js (Coming soon)
  13. Add Hours to a Date in JavaScript or Node.js (Coming soon)
  14. Check If a Date Is Today in JavaScript or Node.js (Coming soon)
  15. Check If a Date is Tomorrow in JavaScript or Node.js (Coming soon)
  16. Check If a Date is Yesterday in JavaScript or Node.js (Coming soon)
  17. How to Format a Date YYYY-MM-DD in JavaScript or Node.js (Coming soon)

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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