Node.js：从函数返回值，里面有异步调用

I'm trying to create a synchronous function which creates a random string and checks if there's already a file with that name on Amazon Web Service S3. How could I make the function synchronous as inside of it is the asynchronous web service call to AWS? If the filename already exists, the function should call itself again (recursive) until a available filename is found.I'm trying to create a synchronous function whi