Apply Transform Over Each Element in Array

Problem

Given an integer array arr and a mapping function fn, return a new array with a transformation applied to each element.

The returned array should be created such that returnedArray[i] = fn(arr[i], i).

Please solve it without the built-in Array.map method.

Example 1:

Input: arr = [1,2,3], fn = function plusone(n) { return n + 1; }
Output: [2,3,4]
Explanation:
const newArray = map(arr, plusone); // [2,3,4]
The function increases each value in the array by one.

Example 2:

Input: arr = [1,2,3], fn = function plusI(n, i) { return n + i; }
Output: [1,3,5]
Explanation: The function increases each value by the index it resides in.

Example 3:

Input: arr = [10,20,30], fn = function constant() { return 42; }
Output: [42,42,42]
Explanation: The function always returns 42.

Constraints:

0 <= arr.length <= 1000
-109 <= arr[i] <= 109
fn returns a number

Solution (Javascript)

/**
 * @param {number[]} arr
 * @param {Function} fn
 * @return {number[]}
 */
var map = function (arr, fn) {
  return arr.reduce((transformedArr, element, index) => {
    transformedArr[index] = fn(element, index);
    return transformedArr;
  }, []);
};

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(n).