The purpose of the code is to demonstrate how cryptography can be implemented in Node.js using the crypto module, and how to save and load encrypted data to and from a file.
The code uses the Advanced Encryption Standard (AES) algorithm to encrypt and decrypt a message, and the Galois/Counter Mode (GCM) to provide authentication and integrity protection. The key is a random secret value that is used to encrypt and decrypt the message.
The cipher object is used to encrypt the message, and the decipher object is used to decrypt it. The update and final methods are used to encrypt and decrypt the message in chunks, and the getAuthTag and setAuthTag methods are used to retrieve and set the authentication tag, which is used to verify the integrity of the message.
The code also uses the fs module to read and write files, and the Buffer class to handle binary data. It saves the encrypted data and key to a file, and then loads them back in to decrypt the message.