Coding Challenge Practice - Question 68

The task is to implement a function to determine if a number is prime.

The boilerplate code

function isPrime(num) {
  // your code here
}

If the number is less than or equal to 1, it's not prime.

if(num <= 1) return false;

The number 2 is the only even prime number.

if(num === 2) return true;

If the number is even and greater than 2, then it's not prime.

if(num % 2 === 0) return true; 

For other numbers, divide by odd numbers starting from 3 up until the square root of the number (because if a number has a divisor, any number bigger than the square root will have a corresponding value lesser than the square root). If none of the checked numbers divide it evenly, then the number is prime.

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

The final code

function isPrime(num) {
  // your code here
  if(num <= 1) return false;
  if(num === 2) return true;
  if(num % 2 === 0) return false;

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

That's all folks!