Tiny TypeScript Random ID Generator (with crypto)

const URL64 = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-_' as const
 
type RandomIdOptions = {
  length?: number 
  alphabet?: string 
}
 
function getCrypto(): Crypto | undefined {
  try {
    const c = (globalThis as any)?.crypto
    return c && typeof c.getRandomValues === 'function' ? c : undefined
  } catch {
    return undefined
  }
}
 
function fillRandom(bytes: Uint8Array): void {
  const c = getCrypto()
  if (c) {
    c.getRandomValues(bytes)
    return
  }
  for (let i = 0; i < bytes.length; i++) bytes[i] = (Math.random() * 256) | 0
}
 
function nextPow2Mask(n: number): number {
  const p = 1 << Math.ceil(Math.log2(n))
  return p - 1
}
 
export function randomId(opts?: number | RandomIdOptions): string {
  const length = typeof opts === 'number' ? opts : opts?.length ?? 16
  const alphabet = (typeof opts === 'object' && opts?.alphabet) || URL64
 
  if (length <= 0) return ''
  if (alphabet.length <= 1) throw new Error('alphabet must have length >= 2')
 
  const mask = nextPow2Mask(alphabet.length)
  const isPow2 = (alphabet.length & (alphabet.length - 1)) === 0
 
  const out: string[] = new Array(length)
 
  if (isPow2) {
    const bytes = new Uint8Array(length)
    fillRandom(bytes)
    for (let i = 0; i < length; i++) {
      out[i] = alphabet[bytes[i] & mask]
    }
    return out.join('')
  }
 
  const step = Math.ceil(length * 1.6)
  let i = 0
  while (i < length) {
    const buf = new Uint8Array(step)
    fillRandom(buf)
    for (let j = 0; j < buf.length && i < length; j++) {
      const idx = buf[j] & mask
      if (idx < alphabet.length) {
        out[i++] = alphabet[idx]
      }
    }
  }
  return out.join('')
}
 
// Examples
// 16‑char URL‑safe ID
console.log(randomId())
 
// 10‑char base32 ID (lowercase letters + digits, 32‑char alphabet)
console.log(randomId({ length: 10, alphabet: 'abcdefghijklmnopqrstuvwxyz234567' }))
 
// 21‑char Nano‑style alphabet (non‑power‑of‑two, unbiased via rejection sampling)
console.log(randomId({ length: 21, alphabet: '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_' }))