callpack

Simply wraps a callback to "pack" multiple values into a single object.

Usage

Callpack packs values into an array-like object by default.

callpack(function(err, result) {
  console.log(result[0] === 1);       // true
  console.log(result[1] === 2);       // true
  console.log(result.length);         // 2
  console.log(Array.isArray(result)); // false
  console.log(Array.from(result));    // [1, 2]
  console.log(result.toString());     // "[object Pack]"
})(null, 1, 2);

Callpack packs values into a simple object when you provide names.

callpack(function(err, result) {
  console.log(result.first);  // "Bill"
  console.log(result.second); // "Thornton"
}, 'first', 'second')(null, 'Bill', 'Thornton');

Realistic example use case.

var async = require('async');
var callpack = require('callpack');
var fs = require('fs');
var request = require('request');

async.auto({
  'page': cb => request('http://www.google.com', callpack(cb, 'response', 'body')),
  'save': ['page', (result, cb) => {
    if (result.page.response.statusCode == 200) {
      fs.writeFile('./index.html', result.page.body, cb);
    } else {
      cb(result.page.response.statusMessage);
    }
  }]
});

Promisifying a callback library.

var callpack = require('callpack');
var promisify = require('es6-promisify');
var _request = require('request');
var request = promisify(function() {
  var cbIndex = arguments.length - 1;
  arguments[cbIndex] = callpack(arguments[cbIndex], 'response', 'body');

  _request.apply(_request, arguments);
});

request('http://www.google.com').then(result => console.log(result.body), console.error);

Reasoning

Consuming an asynchronous function allows for flexibility, but often tools like the awesome async library are easier to use with only a single value. How the function is consumed should be up to the developer, not the library.

Thus callpack doesn't make any assumptions like "multiple values means an array". Instead the decision is still up the end developer. You may use the result from callpack as array, convert into an array, or use it to get an object that closely mimics spreading the arguments over a function.