01. Add and Subtract

function solve(arr) {

    let originalSum = 0;
    let modifiedSum = 0;

    for (let i = 0; i < arr.length; i++) {
        originalSum += arr[i];
    }
    for (let i = 0; i < arr.length; i++) {
        arr[i] % 2 === 0 ? arr[i] += i : arr[i] -= i;
    }
    for (let i = 0; i < arr.length; i++) {
        modifiedSum += arr[i];
    }
    console.log(arr, `\n` + originalSum, `\n` + modifiedSum);
}


Решение с lambda:

function solve(arr) {

    let originalSum = arr.reduce((a, b) => a + b, 0);

    for (let i = 0; i < arr.length; i++) {
        arr[i] % 2 === 0 ? arr[i] += i : arr[i] -= i;
    }

    let modifiedSum = arr.reduce((a, b) => a + b, 0);

    console.log(arr, `\n` + originalSum, `\n` + modifiedSum);

}