decryption.js

const nacl   = require('tweetnacl');
const util   = require('tweetnacl-util');
const scrypt = require('scryptsy');
const fs     = require('fs');

let password = "Node.js is cool";
// these are our previous salt and nonce from encryption.js:
let salt = new Uint8Array([ 232, 66, 12, 24, 90, 175, 58, 106, 13, 220, 241, 77, 156, 230, 140, 218 ]);
let nonce = new Uint8Array([ 143, 28, 216, 227, 2, 177, 160, 159, 243, 180, 138, 230, 142, 165, 28, 189, 208, 63, 130, 131, 204, 240, 105, 142 ]);

// Generate the key:
let N = 16384; // number of iterations
let r = 8; // Memory factor
let p = 1; // Parallelization factor
let key = scrypt(password, salt, N, r, p, nacl.secretbox.keyLength);

// Get encrypted data from file:
let content = fs.readFileSync('file.txt', 'ascii'); // no need of utf8 with Base64
console.log(content);
let encrypted = util.decodeBase64(content);

// Decrypt the data:
let decrypted = nacl.secretbox.open(encrypted, nonce, key);
console.log(util.encodeUTF8(decrypted)); // function should be named "decodeUTF8", they reversed names!

For encryption:
https://gist.github.com/authmane512/3b14bbc2335bb8ffc30004b29d8b4de5