What is if (isPrime(i)) referencing? and why do for loops no longer make sense?

Tell us what’s happening:
I understand that the helper function only returns a value of prime for sum to use. But is if(isPrime(i)) on line 18 referencing i of the helper function? I think I just don’t understand how for loops work…
The second question (which made complete sense up until now) is based on my iteration() function. A fundamental understanding of both referencing helper functions and for loops would really solidify my understanding of javascript :grinning:
Your code so far

function sumPrimes(num) {
  // Helper function to check primes
  function isPrime(num) {
    for (let i = 2; i <= Math.sqrt(num); i++) {
      //square rooting num so that each iteration of i, which is two, is balanced out by the square root(for example, sqrt of 10 is 3, so i is 6, 10 % 6 is 4, which returns true)?
      if (num % i == 0)
      //this returns false if the remainder of num based on the square root of i is a composite
        return false;
    return true;

  // Check all numbers for prime
  let sum = 0;
  for (let i = 2; i <= num; i++) {
    //we use 10 again, which means i = 20
    if (isPrime(i))
    //we then put in 20 into isPrime(20) which based on the for loop, returns true? then the answer is 21 if I pass in 10 as num? HALP
      sum += i;
  return sum;


function iteration(int){
  let sumnum = 0;
  for(let i = 2; i < int; i++)
    sumnum += i
return sumnum;

/*shouldn't it be 6 because the for loop only iterates 3 times (exluding the int value 4 itself because not <=) and thus sumnum is 3 iterations of i added together, and since i is 2 we return 6.
but console.log() returns 5? It gets even weirder if I increase i in iteration() to 3, and the console logs a lower number of 3. */

  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.128 Safari/537.36.

Challenge: Sum All Primes

Link to the challenge:

No. The i inside of the helper function is in the scope of the helper function. It can’t be accessed outside of the helper function.

The i in isPrime(i) refers to the i in this scope, this for loop over every number from 2 to num.

The idea is to read the function name literally. Loop for i from 2 to num. If i is prime, then add i to the sum.

The body if the isPrime is separate from the use of the function.


Summing from 2 to 4, excluding 4 is 2+3=5. I’m not sure where 6 comes from? Did you start adding at 1 perhaps?

I’m such an idiot. I kept thinking that since I i = 2, i++ means increment it at 2 each time, not by one starting from 2. I also understand why increasing i to 3 decreases the value because if it goes higher than num which is 4 the for loop stops.

Oh, we’ve all misread code from time to time.

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