Promisify: from callback to promise

promisify.js

function promisify(func, scope) {
    return function () {
        var args = [].slice.call(arguments);

        var deferred = Promise.defer();

        args.push(function () {
            if (arguments[0]) {
                // There was an error
                deferred.reject(arguments[0]);
            } else {
                // Success
                deferred.resolve([].slice.call(arguments, 1));
            }
        });

        func.apply(scope || null, args);

        return deferred.promise;
    };
}