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

const reduce = (array, fn, initialValue) => {
  let accumolator = initialValue;

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

  return accumolator;
};

const sum = reduce(arr, (a, b) => a + b, 0);

console.log(sum);