trying to find Prime numbers in an array

Question

I'm wanting to remove the non-prime numbers from an Array, the following is only removing the even numbers instead of the prime numbers.

function sumPrimes(num) {
  //Produce an array containing all number of to and including num
  let numArray = [];
  for (let i = 1; i <= num; i++) {
    numArray.push(i);
  }

  //Remove non-prime numbers from the array     
  numArray.map((number) => {
    for (let i = 2; i < number; i++) {
        if(number % i === 0) {
            let index = numArray.indexOf(number);
            return numArray.splice(index, 1);       
        }
    }   
  });

 return numArray;

}

sumPrimes(10);

This is currently returning:

[1, 2, 3, 5, 7, 9]

However, prime numbers are 1, 2, 3, 5, 7 (not include 9);

Answer 1

Use filter() instead:

 var numArray = [2, 3, 4, 5, 6, 7, 8, 9, 10] numArray = numArray.filter((number) => { for (var i = 2; i <= Math.sqrt(number); i++) { if (number % i === 0) return false; } return true; }); console.log(numArray);

Answer 2

The previous answer doesn't work correctly for negative numbers.

As you can see here , the simplest way to find prime numbers is:

 const array = [-5, -3, -2, -1, ...Array(20).keys()]; // Array(20).keys() generates numbers from 0 to 19. function isPrime(num) { for (let start = 2; num > start; start++) { if (num % start == 0) { return false; } } return num > 1; } console.log(array.filter(isPrime)); // [2, 3, 5, 7, 11, 13, 17, 19]

Answer 3

The answer by Farnaz Kakhsaz works great, but it is slow. You only need to check until the root of 'num'. Thanks!

const array = [-5, -3, -2, -1, ...Array(20000).keys()]; 

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

console.log(array.filter(isPrime));