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Check if a number is prime in JavaScript

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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Problem

Checking for prime numbers is a common task in mathematics and computer science. It's also pretty common in coding interviews and challenges. Let's see how to check if a number is prime and how to generate prime numbers up to a given number.
To check if an integer is a prime number, we can simply use a for loop. Inside the loop we can check if the number is divisible by any number from 2 to the square root of the given number. If such a number is found, we can return false. If no such number is found, we can return true, unless the number is less than 2.
> [!NOTE]
>
> This code snippet may be fairly inefficient for very large numbers. Optimizations are possible, but they are beyond the scope of this article.

Solution

const isPrime = num => {
  const boundary = Math.floor(Math.sqrt(num));
  for (let i = 2; i <= boundary; i++) if (num % i === 0) return false;
  return num >= 2;
};

isPrime(11); // true

> [!NOTE]
>
> This code snippet may be fairly inefficient for very large numbers. Optimizations are possible, but they are beyond the scope of this article.
Using the Sieve of Eratosthenes algorithm, we can generate primes up to a given number. The algorithm works as follows:

Generate an array from 2 to the given number.
Use Array.prototype.filter() to filter out the values divisible by any number from 2 to the square root of the provided number.

Code Snippets

const isPrime = num => {
  const boundary = Math.floor(Math.sqrt(num));
  for (let i = 2; i <= boundary; i++) if (num % i === 0) return false;
  return num >= 2;
};

isPrime(11); // true

const primes = num => {
  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
    sqroot = Math.floor(Math.sqrt(num)),
    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
  return arr;
};

primes(10); // [2, 3, 5, 7]

Context

From 30-seconds-of-code: is-prime

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