LeetCode Problem 2635 Apply Transform Over Each Element in Array

I’ve been going through the 30 Days of Javascript in LeetCode. Right now, I’m on 2635. 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.

First Attempt:

When I first encountered the problem, I overthought it, which ended up consuming nearly the entire day.

let fn = function (arr, i = 0) {
  return i + arr;
};

var map = function (arr, fn) {
  let returnedArray = [];
  for (let i = 0; i < arr.length; i++) {
    returnedArray[i] = fn(arr[i], 1);
  }
  return returnedArray;
};

In my first attempt, this is what I did:

I began by defining a function (transformation function) called fn. It takes the parameters arr and i, which is set to 0 if not defined. The function returns the result of adding each item in arr with i.
Next, I created another function called map, which takes arr and the previously defined fn as parameters. I initialized an empty array called returnedArray to store the transformed values.
I then set up a for loop to iterate through each value in the array and add it to returnedArray.
The line returnedArray[i] = fn(arr[i], 1); is equivalent to returnedArray.push(fn(arr[i], 1));. Please excuse any confusion, as I am more familiar with Java.

I thought I had nailed it until I saw this wrong answer show up after running it. It turned out one test case failed.

After spending a while thinking and reviewing the challenge, I noticed they had specified that fn was already provided. All I had to do was adjust this line to returnedArray[i] = fn(arr[i], i). Quite straightforward, isn't it? No need to redefine the fn function. Let's give it another shot.

var map = function (arr, fn) {
  let returnedArray = [];
  for (let i = 0; i < arr.length; i++) {
    returnedArray.push(fn(arr[i], i));
  }
  return returnedArray;
};

And this, my friends, worked.

LeetCode Problem 2635 Apply Transform Over Each Element in Array — LeetCode: 30 Days of JavaScript

Problem: