How to use the tweetnacl.secretbox.keyLength function in tweetnacl

// @flow

import { randomBytes, secretbox } from 'tweetnacl'
import { encodeUTF8, decodeUTF8, encodeBase64, decodeBase64 } from 'tweetnacl-util'

const NONCE_LEN = secretbox.nonceLength
const KEY_LEN = secretbox.keyLength

type Utf8 = string
type Base64 = string

const generateCredentials: () => { nonce: Uint8Array, key: Uint8Array }
= () => ({
  nonce: randomBytes(NONCE_LEN),
  key: randomBytes(KEY_LEN),
})

const join: (Uint8Array, Uint8Array) => Uint8Array
= (a, b) => {
  const c = new Uint8Array(a.length + b.length)
  c.set(a)
  c.set(b, a.length)
  return c

export const generateRandomKey = () =>
  encodeBase64(randomBytes(secretbox.keyLength));
export const generatePasswordShaKey = (

constructor () {
    this.key = randomBytes(secretbox.keyLength)
  }