Coding Challenge Practice - Question 68

Source: Dev.to

Problem Statement

Implement a function isPrime(num) that returns true if the given number is a prime number and false otherwise.

Approach

Handle edge cases

• Numbers less than or equal to 1 are not prime.
• 2 is the only even prime number.
• Any other even number greater than 2 cannot be prime.

Check odd divisors

For odd numbers greater than 2, test divisibility by odd integers starting from 3 up to the square root of the number.

• If any divisor divides the number evenly, the number is not prime.
• If none do, the number is prime.

Code

function isPrime(num) {
  // Edge cases
  if (num <= 1) return false;   // 0, 1, and negatives are not prime
  if (num === 2) return true;   // 2 is the only even prime
  if (num % 2 === 0) return false; // other even numbers are not prime

  // Check odd divisors up to sqrt(num)
  const limit = Math.sqrt(num);
  for (let i = 3; i <= limit; i++) {
    if (num % i === 0) return false;
  }
  return true;
}