Function to iterate through nested arrays and find largest values in each.

largestOfFour.js

function largestOfFour(arr) {
    for (var i = 0; i < arr.length; i++) {
        for (var j = 0; j < arr[i].length; j++) {
            arr[i] = Math.max(...arr[i]);
        }
    }
    return arr;
}
largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);