example using node.js crypto API with aes-256-gcm

crypto-aes-256-gcm-demo.js

const buffer = require('buffer');
const crypto = require('crypto');

// Demo implementation of using `aes-256-gcm` with node.js's `crypto` lib.
const aes256gcm = (key) => {
  const ALGO = 'aes-256-gcm';

  // encrypt returns base64-encoded ciphertext
  const encrypt = (str) => {
    // The `iv` for a given key must be globally unique to prevent
    // against forgery attacks. `randomBytes` is convenient for
    // demonstration but a poor way to achieve this in practice.
    //
    // See: e.g. https://csrc.nist.gov/publications/detail/sp/800-38d/final
    const iv = new Buffer(crypto.randomBytes(12), 'utf8');
    const cipher = crypto.createCipheriv(ALGO, key, iv);

    // Hint: Larger inputs (it's GCM, after all!) should use the stream API
    let enc = cipher.update(str, 'utf8', 'base64');
    enc += cipher.final('base64');
    return [enc, iv, cipher.getAuthTag()];
  };

  // decrypt decodes base64-encoded ciphertext into a utf8-encoded string
  const decrypt = (enc, iv, authTag) => {
    const decipher = crypto.createDecipheriv(ALGO, key, iv);
    decipher.setAuthTag(authTag);
    let str = decipher.update(enc, 'base64', 'utf8');
    str += decipher.final('utf8');
    return str;
  };

  return {
    encrypt,
    decrypt,
  };
};

const KEY = new Buffer(crypto.randomBytes(32), 'utf8');

const aesCipher = aes256gcm(KEY);

const [encrypted, iv, authTag] = aesCipher.encrypt('hello, world');
const decrypted = aesCipher.decrypt(encrypted, iv, authTag);

console.log(decrypted); // 'hello, world'

Most applications should consider using the new KeyObject API instead of passing keys as strings or 
Buffers due to improved security features. -https://nodejs.org/api/crypto.html#class-keyobject

e.g. generateKey('aes', { length: 256 }

crypto.generateKey('aes', { length: 256 }) - What exactly do you want to replace with this?