javascript how to tell prime number code example

Example 1: javascript find a prime number

// This seems to be the most common and comprehensive means
// of implementing a 'is a number a prime function' in javascript 
// if you would like t0 learn more complex , but efficient algorithms. 
// Visit the link below
// https://stackoverflow.com/questions/11966520/how-to-find-prime-numbers-between-0-100

function isPrime(num) {
  if(num < 2) return false;

  for (let k = 2; k < num; k++){
    if( num % k == 0){
      return false;
    }
  }
  return true;
}

Example 2: javascript find prime number

function test_prime(n){
  if (n===1) {
    return false;
  }else if(n === 2){
    return true;
  }else{
    for(var x = 2; x < n; x++){
      if(n % x === 0){
        return false;
      }
    }
    return true;  
  }
}

console.log(test_prime(37));

Example 3: Javascript prime number check

// 6k+-1 optimisation
function is_prime (n) {
  if (n < 3) return n > 1;
  else if (n % 2 === 0 || n % 3 === 0) return false;
  else if (n < 25) return true;
  let i = 5;
  while (i * i <= n ) {
    if (n % i === 0 || n % (i + 2) === 0) return false;
    i += 6;
  }
  return true;
}