checkPrime()

Step 1: Understand the problem statement

The first step in solving this problem is to understand what a prime number is. A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. The function check-prime should therefore determine whether a given number is prime or not.

function checkPrime(num) {
  // We will implement our code inside this function
}

Step 2: Handle edge cases

We know that numbers less than 2 are not prime so we can handle this edge case first. If the input number is less than 2, our function should return false.

function checkPrime(num) {
  if (num < 2) {
    return false;
  }
}

Step 3: Check divisibility

Next we need to check if the number is divisible by any other number apart from itself and 1. We'll create a loop from 2 up to the square root of the number. If our number is divisible by any of these numbers then it is not prime and we should return false.

function checkPrime(num) {
  if (num < 2) {
    return false;
  }
  for(let i = 2; i <= Math.sqrt(num); i++) {
    if (num % i === 0) {
      return false;
    }
  }
}

Step 4: Return true when number is a prime number

If the number is not divisible by any number in our loop then it must be prime. At the end of our function we will therefore add a line to return true.

function checkPrime(num) {
  if (num < 2) {
    return false;
  }
  for(let i = 2; i <= Math.sqrt(num); i++) {
    if (num % i === 0) {
      return false;
    }
  }
  return true;
}

Conclusion

The function checkPrime successfully checks whether a number is prime or not using a simple for loop and the modulus operator to check divisibility. Remember that we only needed to loop up to the square root of the number because a larger factor of the number would be a multiple of smaller factor that has already been checked.