JS: Generate a random string from a given characterset using only each character once

index.js

function generateUnique(length) {
  const generated = []
  const characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'

  for (let i = 0; i < length; i++) {
    let character = ''

    do {
      character = characters[random(characters.length)]
    } while (generated.includes(character))

    generated.push(character)
  }

  return generated.join('')
}

function random(max) {
  return Math.floor(Math.random() * max)
}