const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];

const filter = (array, fn) => {
  const newArr = [];

  for (let i = 0; i < array.length; i++) {
    if (fn(array[i])) {
      newArr.push(array[i]);
    }
  }

  return newArr;
};

const evenNumbers = filter(arr, (num) => num % 2 === 0);

console.log(evenNumbers);