🔢 Fast Prime Gen

Generate sequential prime numbers, lazily and forever.

A tiny, dependency-free package that streams prime numbers on demand. Instead of precomputing a list, it yields the next prime using a postponed Sieve of Eratosthenes (based on Vladimir Agafonkin's work) - so memory grows with the primes you've actually seen, not a fixed upper bound.

• 🪶 Zero dependencies
• ♾️ Lazy & infinite - pull one prime or a million
• 🧩 Standard generator - works with for…of, spread, destructuring
• 🔒 Typed - ships .d.ts, with ESM and CommonJS builds

Install

npm install fast-prime-gen

Quick start

import { primes } from 'fast-prime-gen';

const gen = primes();

gen.next().value;  // 2
gen.next().value;  // 3
gen.take(3);       // [5, 7, 11]  → continues from where you left off

primes().skip(100).next().value; // 547  → fresh generator, jump ahead

const { primes } = require('fast-prime-gen');

console.log(primes().take(5)); // [2, 3, 5, 7, 11]

Because it's a plain generator, it drops into any iterable context:

for (const p of primes()) {
  if (p > 50) break;
  console.log(p); // 2, 3, 5, 7, … 47
}

API

primes(): PrimeGenerator

A lazy, never-ending generator of sequential primes, plus the chainable helpers below.

.skip(amount = 1)

Advances past amount primes and returns the same generator for chaining. 0 is a no-op.

primes().skip(100).next().value; // 547

⚠️ Throws RangeError if amount isn't a non-negative integer.

.take(amount): number[]

Collects the next amount primes into an array. Composes with .skip().

primes().take(5);         // [2, 3, 5, 7, 11]
primes().skip(4).take(3); // [11, 13, 17]

⚠️ Throws RangeError if amount isn't a non-negative integer.

primesBelow(limit): number[]

Every prime strictly below limit (exclusive bound).

primesBelow(20); // [2, 3, 5, 7, 11, 13, 17, 19]

⚠️ Throws TypeError for a non-number, or RangeError if limit > Number.MAX_SAFE_INTEGER.

Limits

Primes use JavaScript number arithmetic, exact only up to Number.MAX_SAFE_INTEGER (2⁵³ − 1). Past that point the generator throws a RangeError rather than silently emit wrong values. Larger primes would need a bigint variant (not currently provided).

Migrating from 1.x

| 1.x | 2.x | | --- | --- | | const PostponedSieve = require('fast-prime-gen') | const { primes } = require('fast-prime-gen') | | JavaScript source, no types | TypeScript source, ships .d.ts | | skip() silently ignored bad input | skip() throws on non-integer / negative | | skip() only | added take() and primesBelow() | | wrong values past 2⁵³ | throws past Number.MAX_SAFE_INTEGER |

Development

npm install
npm test          # Vitest suite
npm run bench     # benchmarks
npm run lint      # Biome lint + format check  (lint:fix to auto-fix)
npm run typecheck # tsc --noEmit
npm run build     # dist/ → ESM + CJS + .d.ts  (tsup)

License

MIT