CodeSignal - Replace Middle

replaceMiddle.js

/*
We define the middle of the array arr as follows:

if arr contains an odd number of elements, its middle is the element whose index number is the same when counting from the beginning of the array and from its end;
if arr contains an even number of elements, its middle is the sum of the two elements whose index numbers when counting from the beginning and from the end of the array differ by one.
Given array arr, your task is to find its middle, and, if it consists of two elements, replace those elements with the value of middle. Return the resulting array as the answer.

Example

For arr = [7, 2, 2, 5, 10, 7], the output should be
replaceMiddle(arr) = [7, 2, 7, 10, 7].

The middle consists of two elements, 2 and 5. These two elements should be replaced with their sum, i.e. 7.

For arr = [-5, -5, 10], the output should be
replaceMiddle(arr) = [-5, -5, 10].

The middle is defined as a single element -5, so the initial array with no changes should be returned.
*/

function replaceMiddle(arr) {
    let replacementNumber = 0;
    let middle1 = 0;
    let middle2 = 0
    // If odd
    if(arr.length % 2 !== 0) {
        return arr;
    }
    // If even
    else {
        // If even length, divide by two to get first of the two middle numbers, then divide length by 2 and add 1 to get         // second middle number
        middle1 = arr.length / 2;
        middle2 = arr.length / 2 - 1;
        // Sum the two middle numbers
        replacementNumber = arr[middle1] + arr[middle2];
        // Splice out both middle1 and middle2 and replace them with replacementNumber
        arr.splice(middle2, 2, replacementNumber);
        return arr;
    }
}