en.javascript.info/1-js/4-data-structures/7-array/10-maximal-subarray/_js.view/solution.js

function getMaxSubSum(arr) {
  var maxSum = 0,
    partialSum = 0;
  for (var i = 0; i < arr.length; i++) {
    partialSum += arr[i];
    maxSum = Math.max(maxSum, partialSum);
    if (partialSum < 0) partialSum = 0;
  }
  return maxSum;
}