Node.js Creating a Node.js Library that Supports Both Promises and Error-First Callbacks Example Module and Corresponding Program using Bluebird


Example

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);
  }

})();