A JavaScript function takes in a 2D array and adds the diagonal values each way. It then compares the absolute value of both.

diagonalDiff.js

/*
 * Complete the 'diagonalDifference' function below.
 *
 * The function is expected to return an INTEGER.
 * The function accepts 2D_INTEGER_ARRAY arr as parameter.
 *
 */

function diagonalDifference(arr) {
  // Loop over the array both ways
  // Add the values to each result
  // Return the absolute value of the final result
  let forwardResult = 0;
  let backwardResult = 0;

  for (let i = 0; i < arr.length; i++) {
    for (let j = 0; j < arr[i].length; j++) {
      if (i === j) {
        forwardResult += arr[i][j];
      }
      if (i + j === arr.length - 1) {
        backwardResult += arr[i][j];
      }
    }
  }

  const finalResult = Math.abs(forwardResult - backwardResult);
  return finalResult;
}