Typed promisify

export function promisify<T>(thisArg : any, f : Function, ...args):Promise<T>{
    return new Promise<T>(function(resolve, reject) {
        f.call(thisArg, ...args, (err, ret) => {
            if (err) {
                reject(err);
            } else {
                resolve(ret);
            }
        });
    });
}

//Usage example:
//return await promisify<User>(User, User.findByUsername, username);