if conditional when finding prime number - always returning false

Question

var enteredValue = prompt("enter a number");
enteredValue = enteredValue + 0;
console.log(isPrime(enteredValue));

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

Can anyone tell me what I'm doing wrong? The code is always returning false.

`return` ends your loop - and your function. – connexo Mar 04 '18 at 15:51 — connexo, Mar 04 '18 at 15:51

score 2 · Answer 1 · answered Mar 04 '18 at 15:46

You need to move the return of true out side of the loop, because you need to check all factors before returning true.

var enteredValue = +prompt("enter a number");
console.log(isPrime(enteredValue));

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

score 0 · Answer 2 · answered Mar 04 '18 at 15:50

Your method should return true outside of your for loop. With your example you are retuening in first iteratin, by entering else block.

This will work:

function isPrime(num) {
   for (var i = 2; i < num; i++) {
     if (num % i === 0) {
       return false; // return if  it's not a prime
     }
   }
   return true; // return only if it's a prime number
}

You can also check other prime solutions in this post.

if conditional when finding prime number - always returning false

2 Answers2