# Number prime test in JavaScript

I’m trying to complete the Codewars challenge that asks you to check if a number is a prime number. For whatever reason, my solution doesn’t seem to work for the square of odd prime numbers (e.g. `9` returns `true` instead of `false`).

```function isPrime(num) {

if (num === 2) {
return true;
} else if (num > 1) {
for (var i = 2; i < num; i++) {

if (num % i !== 0) {
return true;
} else if (num === i * i) {
return false
} else {
return false;
}
}
} else {
return false;
}

}

console.log(isPrime(121));```

P.s. I included that second else/if statement because I was trying to solve the problem.

As simple as possible:

```function isPrime(num) {
for(var i = 2; i < num; i++)
if(num % i === 0) return false;
return num > 1;
}
```

With the ES6 syntax:

```const isPrime = num => {
for(let i = 2; i < num; i++)
if(num % i === 0) return false;
return num > 1;
}
```

You can also decrease the complexity of the algorithm from `O(n)` to `O(sqrt(n))` if you run the loop until square root of a number:

```const isPrime = num => {
for(let i = 2, s = Math.sqrt(num); i <= s; i++)
if(num % i === 0) return false;
return num > 1;
}
```