JavaScript Function#makeCallbackable

function-makecallbackable.js

/*
 * Function#makeCallbackable
 *
 * Copyright (c) 2012 "Cowboy" Ben Alman
 * Licensed under the MIT license.
 * http://benalman.com/about/license/
 */

Function.prototype.makeCallbackable = function() {
  var fn = this;
  return function callbackable() {
    var result = fn.apply(this, arguments);
    // If more args were specified than fn accepts, and the last argument is
    // a function, let's assume it's an async "done" function and call it.
    var length = arguments.length;
    var done = arguments[length - 1];
    if (length > fn.length && typeof done === 'function') {
      done(result);
    }
  };
};

usage.js

// An asynchronous function.
function async(a, b, done) {
  done(a + b);
}

// A synchronous function.
function sync(a, b) {
  return a + b;
}

var async2 = async.makeCallbackable();
var async3 = sync.makeCallbackable();

async(1, 2, console.log.bind(console));
// logs 3

async2(1, 2, console.log.bind(console));
// logs 3

async3(1, 2, console.log.bind(console));
// logs 3

Aren’t the functions returned by makeAsync still synchronous? Of course, that’s fine if the goal is to just turn a given function into one with an async-like signature.

Derp. Changing it to Function#makeCallbackable.