JavaScript Program to Check Whether a Number is Prime

A prime number is a whole number greater than 1 that has no divisors other than 1 and itself — 2, 3, 5, 7, 11, 13... are all prime, while 4, 6, 8, 9 are not. This program checks whether a given number is prime by testing whether any smaller number divides it evenly.

Program to Check Prime Number in JavaScript

function isPrime(num) {
  if (num < 2) return false;

  // Only need to check divisors up to the square root of num
  for (let i = 2; i <= Math.sqrt(num); i++) {
    if (num % i === 0) {
      return false; // found a divisor, so it's not prime
    }
  }
  return true;
}

console.log(isPrime(29));
console.log(isPrime(15));

Output:

true
false

The loop only checks divisors up to Math.sqrt(num) instead of all the way up to num — if a number greater than its square root divided it evenly, there would have to be a matching smaller divisor too, so checking beyond the square root is redundant. This keeps the function fast even for fairly large numbers.

Numbers less than 2 (0, 1, and negatives) are never prime, which is why the function returns false immediately for them, before the loop even has a chance to run — a common edge case this kind of check needs to handle explicitly.

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Printing All Prime Numbers up to N

function printPrimesUpTo(n) {
  const primes = [];
  for (let num = 2; num <= n; num++) {
    if (isPrime(num)) primes.push(num);
  }
  return primes;
}

console.log(printPrimesUpTo(30));

Output:

[ 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ]

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