A helper function which wrap any function expecting callback as the last argument and return promise

promisify.js

function toArray(obj) {
	return [].slice.apply(obj);
}

module.exports = function promisify(fn) {
	return function () {
		const args = toArray(arguments);
		const self = this;

		return new Promise(function (resolve, reject) {
			const fnArgs = args.concat(function (err) {
				if (err) {
					return reject(err);
				}

				const callbackArgs = toArray(arguments);

				callbackArgs.shift();
				resolve.apply(self, callbackArgs);
			});

			fn.apply(self, fnArgs);
		});
	}
};