I'm looking at the very simple demo in the nodejs crypto documentation here:
They very easily demonstrate how to get a shared secret... now what?
How can I use said shared secret to encrypt and decrypt data?
Diffie-Hellman is a key exchange algorithm. It does not provide encryption by itself.
After both parties have established a common secret through D-H, you can use that as a key in a symmetrical encryption algorithm like AES.
The secret can be used for example as a password for https://nodejs.org/api/crypto.html#crypto_crypto_createcipher_algorithm_password
Mind the security note at the end.