Return largest numbers from sub-arrays (JavaScript)

returnLargestFromArrays.js

function returnLargest(arr) {
  
  var newArr = [];

  for (var i = 0; i < arr.length; i++) {
    var subArray = arr[i];
    var largest = arr[i][0];
    for (var j = 0; j < arr[i].length; j++) {
      if (subArray[j] > largest) {
        largest = subArray[j];
      }
    }
    newArr.push(largest);
  }
  return newArr;
}

returnLargest([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);