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Prime Time: Diving Into Prime Numbers with JavaScript

Hey there, fellow coders! If you’ve ever had a math class, you’re probably familiar with prime numbers—those elusive integers greater than 1 that have exactly two distinct positive divisors: 1 and the number itself. They’re like the celebrities of the number world, unique and indivisible!

In the realm of programming, prime numbers come up more often than you’d think. Whether you’re dealing with cryptography, optimization problems, or just a quirky coding challenge, knowing how to handle primes in JavaScript is a nifty trick to have up your sleeve. So, let’s roll up our sleeves and get our hands dirty with some prime number algorithms in good ol’ JS.

The Prime Directive: Identifying Prime Numbers

Before we start crafting functions left and right, let’s lay down the groundwork. A prime number checker is a function that takes an integer and returns true if it’s prime, and false otherwise. Sounds simple enough, right? Well, here’s a basic implementation to get us started:

function isPrime(num) {
  if (num <= 1) return false; // negatives, 0 and 1 are not prime
  if (num <= 3) return true; // 2 and 3 are prime

  // If divisible by 2 or 3, it's not prime
  if (num % 2 === 0 || num % 3 === 0) return false;

  // Check for divisibility by all numbers up to the square root of num
  for (let i = 5; i * i <= num; i += 6) {
    if (num % i === 0 || num % (i + 2) === 0) return false;

  return true;

This function is a decent start, but let’s remember that efficiency is key, especially when dealing with large numbers. The above code already does a smart thing by only checking up to the square root of the number. Why? Because if num is divisible by some number p, then it’s divisible by num / p as well—and one of those divisors must be less than or equal to the square root of num.

Sieve of Eratosthenes: Sieving Prime Numbers Efficiently

When it comes to generating a list of prime numbers up to a certain limit, the Sieve of Eratosthenes algorithm is a classic. It’s an ancient, efficient way to find all primes smaller than a given number. Here’s the lowdown in JavaScript:

function sieveOfEratosthenes(limit) {
  let primes = Array(limit + 1).fill(true); // Initialize array to keep track of prime numbers
  primes[0] = primes[1] = false; // 0 and 1 are not primes

  for (let i = 2; i <= Math.sqrt(limit); i++) {
    if (primes[i]) {
      // Mark multiples of i as non-prime
      for (let j = i * i; j <= limit; j += i) {
        primes[j] = false;

  // Extract the prime numbers
  return primes.reduce((acc, val, index) => {
    if (val) acc.push(index);
    return acc;
  }, []);

With this function, you’re essentially marking the multiples of each number as non-prime, starting from 2. The beauty of this algorithm is that it’s super efficient for finding all primes less than a given number n, with a complexity of O(n log log n).

Prime Factorization: Breaking It Down

Sometimes, you might need to break down a number into its prime factors. This is a fundamental concept in number theory, and it’s pretty handy in programming too. Here’s a function that returns an array of the prime factors of a given number:

function primeFactors(num) {
  let factors = [];
  // Handle 2s that divide num
  while (num % 2 === 0) {
    num /= 2;

  // Now check for odd numbers up to the square root of num
  for (let i = 3; i * i <= num; i += 2) {
    while (num % i === 0) {
      num /= i;

  // If num is a prime number greater than 2, push it to factors
  if (num > 2) factors.push(num);

  return factors;

Wrapping Up the First Half

We’ve covered the basics of identifying, generating, and factorizing prime numbers in JavaScript. You’ve got a solid foundation with the isPrime function, a classic algorithm with the sieveOfEratosthenes, and a handy primeFactors function for decomposition.

Stay tuned for the second half of the article where we’ll dive into some advanced topics like optimizing our prime-checking function and exploring more complex use cases. We’ll also look at how to handle really large numbers that might make our current functions break a sweat.

Until then, keep those prime numbers rolling, and may your code be as clean and efficient as the primes themselves!

Welcome back, prime enthusiasts! We’ve got the basics down, but what about when the going gets tough? Large numbers can be a real pain for our algorithms, so let’s tighten the bolts and give our prime number functions a performance boost.

Wheel Factorization: Skipping the Obvious

Wheel factorization is a technique to skip checking multiples of the first few primes. It’s like a Sieve of Eratosthenes, but on steroids. By ignoring numbers we know can’t be prime (like those divisible by 2, 3, or 5), we can save ourselves a bunch of work. Here’s how you might implement a simple wheel to skip even numbers:

function isPrimeOptimized(num) {
  if (num <= 1) return false;
  if (num <= 3) return true;
  if (num % 2 === 0 || num % 3 === 0) return false;

  for (let i = 5; i * i <= num; i += 6) {
    if (num % i === 0 || num % (i + 2) === 0) return false;
  return true;

This function is already using a 2-wheel by checking i and i + 2. You can extend this to a 3-wheel or 5-wheel by adding more conditions and skips, but be careful—complexity can increase quickly!

BigNums: When Numbers Outgrow Standard Types

JavaScript’s standard number type can only safely represent integers up to 2^53 – 1 (Number.MAXSAFEINTEGER). For anything larger, you’ll need to use the BigInt type. Here’s how you might adjust our isPrime function to use BigInt:

function isPrimeBigInt(num) {
  let n = BigInt(num);
  if (n <= 1n) return false;
  if (n <= 3n) return true;
  if (n % 2n === 0n || n % 3n === 0n) return false;

  for (let i = 5n; i * i <= n; i += 6n) {
    if (n % i === 0n || n % (i + 2n) === 0n) return false;
  return true;

Remember that BigInt operations can be slower than standard numbers, so use them wisely!

Memoization: Remembering the Past

Memoization is a technique where we store the results of expensive function calls and return the cached result when the same inputs occur again. For prime numbers, we could memoize the results of our isPrime function to avoid recalculating:

const primeCache = new Map();

function isPrimeMemoized(num) {
  if (primeCache.has(num)) return primeCache.get(num);

  let isPrimeVal = isPrimeOptimized(num); // Use the optimized version we created
  primeCache.set(num, isPrimeVal);
  return isPrimeVal;

This can vastly increase performance when checking the same numbers repeatedly.

Advanced Algorithms: The Next Level

There are more advanced algorithms like the Miller-Rabin primality test or the AKS primality test, which can handle large numbers more efficiently and deterministically. While these are beyond the scope of this article, it’s worth knowing they exist for when you need to pull out the big guns.

JavaScript Libraries: Standing on the Shoulders of Giants

Sometimes, it’s best to use tools that others have sharpened. For prime number generation and testing, you might want to check out libraries like prime-number or big-integer on npm. These libraries have optimized algorithms and can handle large numbers with ease.

For example, the prime-number library can be used as follows:

const prime = require('prime-number');

console.log(prime(17)); // true

And for big integers, you might use big-integer like this:

const bigInt = require("big-integer");

let largeNumber = bigInt("1234567891011121314151617181920");
console.log(largeNumber.isPrime()); // false

Conclusion: The Prime End

We’ve traversed the landscape of prime numbers in JavaScript, from simple checks to advanced optimizations and libraries. Whether you’re dealing with small-scale applications or massive numerical computations, the techniques and tools we’ve discussed will help you handle prime numbers with confidence and efficiency.

Remember, the key to mastering primes (and programming in general) is to keep exploring and experimenting. The world of numbers is vast and full of secrets waiting to be unlocked. So go forth, use these prime strategies, and may your code be as robust and elegant as the primes themselves!