Creating a Node.js Library that Supports Both Promises and Error-First Callbacks

Introduction#

Many people like working with promises and/or async/await syntax, but when writing a module it would be useful to some programmers to support classic callback style methods as well. Rather than creating two modules, or two sets of functions, or having the programmer promisify your module, your module can support both programming methods at one using bluebird’s asCallback() or Q’s nodeify().

Example Module and Corresponding Program using Bluebird

math.js

'use strict';

const Promise = require('bluebird');

module.exports = {

  // example of a callback-only method
  callbackSum: function(a, b, callback) {
    if (typeof a !== 'number')
      return callback(new Error('"a" must be a number'));
    if (typeof b !== 'number')
      return callback(new Error('"b" must be a number'));

    return callback(null, a + b);
  },

  // example of a promise-only method
  promiseSum: function(a, b) {
    return new Promise(function(resolve, reject) {
      if (typeof a !== 'number')
        return reject(new Error('"a" must be a number'));
      if (typeof b !== 'number')
        return reject(new Error('"b" must be a number'));
      resolve(a + b);
    });
  },

  // a method that can be used as a promise or with callbacks
  sum: function(a, b, callback) {
    return new Promise(function(resolve, reject) {
      if (typeof a !== 'number')
        return reject(new Error('"a" must be a number'));
      if (typeof b !== 'number')
        return reject(new Error('"b" must be a number'));
      resolve(a + b);
    }).asCallback(callback);
  },

};

index.js

'use strict';

const math = require('./math');


// classic callbacks

math.callbackSum(1, 3, function(err, result) {
  if (err)
    console.log('Test 1: ' + err);
  else
    console.log('Test 1: the answer is ' + result);
});

math.callbackSum(1, 'd', function(err, result) {
  if (err)
    console.log('Test 2: ' + err);
  else
    console.log('Test 2: the answer is ' + result);
});


// promises

math.promiseSum(2, 5)
.then(function(result) {
  console.log('Test 3: the answer is ' + result);
})
.catch(function(err) {
  console.log('Test 3: ' + err);
});

math.promiseSum(1)
.then(function(result) {
  console.log('Test 4: the answer is ' + result);
})
.catch(function(err) {
  console.log('Test 4: ' + err);
});


// promise/callback method used like a promise

math.sum(8, 2)
.then(function(result) {
  console.log('Test 5: the answer is ' + result);
})
.catch(function(err) {
  console.log('Test 5: ' + err);
});


// promise/callback method used with callbacks

math.sum(7, 11, function(err, result) {
  if (err)
    console.log('Test 6: ' + err);
  else
    console.log('Test 6: the answer is ' + result);
});


// promise/callback method used like a promise with async/await syntax

(async () => {

  try {
    let x = await math.sum(6, 3);
    console.log('Test 7a: ' + x);

    let y = await math.sum(4, 's');
    console.log('Test 7b: ' + y);

  } catch(err) {
    console.log(err.message);
  }

})();