encrypt-tweetnacl.js

const nacl = require('tweetnacl')
nacl.util = require('tweetnacl-util')

// generating key pairs
const bob = nacl.box.keyPair()
const alice = nacl.box.keyPair()

// generating one time nonce for encryption
const nonce = nacl.randomBytes(24)
// message for Alice
const utf8 = 'Hello Alice'
// Bob encrypts message for Alice
const box = nacl.box(
  nacl.util.decodeUTF8(utf8),
  nonce,
  alice.publicKey,
  bob.secretKey
)
 
// somehow send this message to Alice
const message = {box, nonce} 

Generating key pairs:

const { publicKey, secretKey } = nacl.box.keyPair()