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Generate secure random values

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Session tokens, password reset links, and API keys must be impossible to guess. Math.random is not suitable for secrets because its output is predictable from earlier outputs. The built-in Web Crypto API provides a cryptographically secure random source instead.

Fill a typed array with random bytes from the secure random source.
const bytes = crypto.getRandomValues(new Uint8Array(16));
console.log(bytes); // e.g. Uint8Array(16) [ 81, 186, 47, ... ]
Generate a v4 UUID, handy as a unique identifier.
console.log(crypto.randomUUID()); // e.g. 721475a3-3280-4f61-8d5f-374c83a7cdfa
For tokens that travel in URLs or headers, encode random bytes as text. 32 random bytes give 256 bits of entropy, plenty for a session token.
const token = crypto.getRandomValues(new Uint8Array(32));
const options = { alphabet: "base64url", omitPadding: true } as const;
console.log(token.toBase64(options)); // e.g. QIUpF1ewQ-bc...
console.log(token.toHex()); // e.g. 4085291757b0...
A uniform random integer in a range takes more care. The naive approach, value % range, is biased: 2 ** 32 is rarely a multiple of the range, so the leftover values at the top wrap around and make small results slightly more likely. Rejection sampling discards that uneven tail and retries, which keeps every value in the range equally likely.
function randomInt(min: number, max: number): number {
  const range = max - min;
  const limit = Math.floor(2 ** 32 / range) * range;
  const sample = new Uint32Array(1);
  do {
    crypto.getRandomValues(sample);
  } while (sample[0] >= limit);
  return min + (sample[0] % range);
}
Roll a die: a uniform integer from 1 to 6.
console.log(randomInt(1, 7)); // e.g. 5

Run this example locally using the Deno CLI:

deno run https://docs.deno.com/examples/scripts/secure_random_values.ts

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