function isPrime(num) {
  if (num < 2) return false;            // 0 and 1 are not prime
  if (num === 2) return true;           // 2 is the only even prime
  if (num % 2 === 0) return false;      // eliminate even numbers
 
  const limit = Math.sqrt(num);         // only check up to sqrt(num)
 
  for (let i = 3; i <= limit; i += 2) { // test only odd divisors
    if (num % i === 0) return false;
  }
 
  return true;
}
 
// Example usage
console.log(isPrime(2));    // true
console.log(isPrime(17));   // true
console.log(isPrime(100));  // false
console.log(isPrime(1));    // false
console.log(isPrime(997));  // true
 

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