A prime number is a whole number greater than 1 with exactly two divisors: 1 and itself. For example, 2 is a prime number because it is only divisible by 1 and 2. In contrast, 4 is not prime since it is divisible by 1, 2 and 4.

Rewrite

`sumPrimes`

so it returns the sum of all prime numbers that are less than or equal to num.

Sometimes I forget not everything needs to be solved within one block of code. If your function tries to solve more than one problem, you should consider divvying up its tasks into other functions.

This problem is actually asking us to do two things: check if the number is prime and add up all the primes numbers up to `num`

. We can do this by using two functions. The first one is to check if the number is prime. It simply returns a boolean of `true`

or `false`

:

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

`isPrime()`

takes `num`

as an argument. It will then run a `for`

loop starting at 2 (because we already know 1 is a prime number). If at any point during our iteration `num`

is divisible by `i`

, we will return `false`

and the function will end there. Otherwise, the number is a prime number and we return `true`

.

This function does exactly what is asked of it: check to see if `num`

is a prime number. It doesn't do any more or any less, however, we still have another function to write. We need to add the sum of all the prime numbers that are less than or equal to `num`

.

```
const sumPrimes = num => {
let primes = []
for (let i = num; i != 1; i--) {
if (isPrime(i)) primes.push(i);
}
return primes.reduce((acc, curr) => acc + curr)
}
```

You can see in this function we are using `isPrime()`

to check if the number, well, is prime. If I were to have combined both of these functions into one it would have been pretty messy.

So, `sumPrimes`

takes in `num`

, then it creates an empty array which we will use to store our prime numbers. A `for`

loop is started but if you look closely, `i`

is instantiated to `num`

and from there we go *down* until `i`

is equal to 0. At this point the loop will end. Inside the loop we are simply checking if `num`

is prime. If it is we add it to our array. If it isn't we do nothing and keep iterating. Our return statement looks fancy schmancy using the `reduce`

method but I only did that to make it look impressive. All it's doing is adding up all of our primes and returning it to us. And that's it!

Like I stated in the first paragraph of this blog post, it's important to know what a problem is asking of you. If you can read between the lines and notice it's wanting you to do more than just one thing, consider if splitting it up into separate functions would be better.

Posted on by:

### Mark Harless

A recent graduate of software engineering immersive boot camp eager to apply creative and freelance skills into the tech industry.

## Discussion