Apply Transform Over Each Element in Array

/*
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.
*/

function map(arr: number[], fn: (n: number, i: number) => number): number[] {
    const res: number[] = [];
    for(let i in arr){
        res.push(fn(arr[i], Number(i)))
    } 
    return res;
};