wrap-as-promise.js

// Omit the last argument (the callback) of the original function and use promise instead.
function wrapAsPromise (func) {
  var expectedArgCount = func.length - 1;
  return function () {
    // Only the last argument (the callback) can be missing.
    switch (true) {
      case arguments.length < expectedArgCount:
        throw new Error('Not enough arguments given. Expecting ' + expectedArgCount + '.');
        break;
      case arguments.length < expectedArgCount:
        throw new Error('Too many arguments given. Expecting ' + expectedArgCount + '.');
        break;
      default:
        // As expected.
        break;
    }

    var _this = this,
        _args = Array.prototype.slice.call(arguments);

    return new Promise(function (resolve, reject) {
      try {
        func.apply(_this, _args.concat(function (err, result) {
          if (err) { reject(err); } else { resolve(result); }
        }));
      } catch (err) {
        reject(err);
      }
    }); 
  }
}