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Challenge 7: Check for Prime Numbers

Write a function that takes a number as input and returns true if it's a prime number and false if it's not. A prime number is a natural number greater than 1 that is not a product of two smaller natural numbers. For example, 2, 3, 5, 7, 11, and 13 are prime numbers, but 4, 6, 8, and 9 are not.

Answer

function isPrime(num) {
  // Check if the number is less than 2, in which case it's not prime
  if (num < 2) {
    return false;
  }

  // Loop through all numbers from 2 up to the square root of the input number
  for (let i = 2; i <= Math.sqrt(num); i++) {
    // If the input number is evenly divisible by any number in this range, it's not prime
    if (num % i === 0) {
      return false;
    }
  }

  // If the function hasn't returned yet, the input number is prime
  return true;
}

Answer Explanation

The function takes a single argument num, which is the number to check for primality. Here's what the function does:

• First, it checks whether the input number is less than 2, which is the smallest prime number. If it is, the function immediately returns false.
• Next, the function loops through all numbers from 2 up to the square root of the input number. This is because any factors of the input number that are larger than its square root must have a corresponding factor that is smaller than its square root, so we only need to check up to the square root.
• For each number in the loop, the function checks whether the input number is evenly divisible by that number. If it is, the function immediately returns false, since the input number is not prime.
• If the loop completes without finding any factors of the input number, the function returns true, indicating that the input number is prime.

Here's an example usage of the function:

console.log(isPrime(2)); // true
console.log(isPrime(3)); // true
console.log(isPrime(4)); // false
console.log(isPrime(5)); // true
console.log(isPrime(6)); // false
console.log(isPrime(7)); // true

In this example, the function correctly identifies the prime numbers 2, 3, 5, and 7 as prime, and the non-prime numbers 4 and 6 as non-prime.