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

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

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

  return newArr;
};

const multipliedNumbers = map(arr, (num) => num * 2);

console.log(multipliedNumbers);