### isPrime

Checks if the provided integer is a prime number.

Check numbers from `2` to the square root of the given number. 
Return `false` if any of them divides the given number, else return `true`, unless the number is less than `2`.

```js
const isPrime = num => {
  const boundary = Math.floor(Math.sqrt(num));
  for (var i = 2; i * i <= boundary; i++) if (num % i == 0) return false;
  return num >= 2;
};
// isPrime(11) -> true
// isPrime(12) -> false
```