Solves the max sub array sum with O(n) time complexity using JavaScript.

maxSubArraySum.js

/**
 * Write a function which accepts an array of integers
 * and a number called n.
 *
 * The function should calculate the maximum sum of n consecutive
 * elements in the array.
 */

const maxSubArraySum = (arr, n) => {
  if (!arr.length) return null;

  let maxSum = 0;
  let tempSum = 0;

  // Loop n times and add those n values together
  // and store value as maxSum
  for (let i = 0; i < n; i++) {
    maxSum += arr[i];
  }

  // Temp sum will be used for comparison later
  tempSum = maxSum;

  for (let i = n; i < arr.length; i++) {
    // Set temp sum to remove the oldest of the n nums
    // and tack on the next num in the array
    // i.e. slide the window over one
    tempSum = tempSum - arr[i - n] + arr[i];

    // Return the largest value
    maxSum = Math.max(maxSum, tempSum);
  }

  return maxSum;
};

console.log(maxSubArraySum([1, 2, 5, 2, 8, 1, 4], 2)); // 10
console.log(maxSubArraySum([1, 2, 5, 2, 8, 1, 4], 4)); // 17
console.log(maxSubArraySum([4, 2, 1, 6], 1)); // 6
console.log(maxSubArraySum([4, 2, 1, 6], 4)); // 13
console.log(maxSubArraySum([], 4)); // null