isPrime

export function isPrime(n: number) {
  if (n < 2) {
    return false;
  }

  if (n % 2 === 0) {
    return n === 2;
  }

  for (let i = 3; i <= Math.sqrt(n); i += 2) {
    if (n % i === 0) {
      return false;
    }
  }

  return true;
}

On the first try we would write the function to check all integers from 2 to n-1. That’s not the most efficient way. We can optimize the function.

Here are the optimizations done in the above function:

1. Except 2, we only have to check if n is divisible by odd numbers only. This optimization cuts the for-loop-cycles in half.
2. We don’t have to check past sqrt(n) for divisors. This will save a lot of wasted cycles.