Tell us what’s happening:
Dont know how to deal with the array fill with negative numbers. My code works for all the other arrays:

largestOfFour([[17, 23, 25, 12], [25, 7, 34, 48], [4, -10, 18, 21], [-72, -3, -17, -10]]) should return [25, 48, 21, -3].
How do i find the biggest out of negative numbers since my max was set to 0 hence it would not be able to work for the negative array?

Your code so far

function largestOfFour(arr) {
// You can do this!
var newA = [];
var max;
for(let i = 0; i < arr.length; i++){
max = arr[i].length;
for(let j = 0; j < arr[i].length; j++){
if(arr[i][j] > max){
max = arr[i][j];
}
}
newA.push(max);
}
console.log(newA);
return newA;
}
largestOfFour([[4, 5, 1, 3], [13, 27, 18, 26], [32, 35, 37, 39], [1000, 1001, 857, 1]]);

You said in your original post that you assign max to zero, but you actually are initializing max to 4 inside the outer for loop. That is the problem of why your function would return [5] for the sub array of [1, 1, 1, 1, 1] instead of [1].

As I already said in my first reply, it is because of the following line of your code.

max = arr[i].length;

You are setting max to be equal to the length of the current sub array being iterated over in arr. My example assumes you call the largestofFour with something like:

Why should it return [1, 2, 3, 4]? Because the largest number in the 1st sub array is 1, the largest number in the 2nd sub array is 2, the largest number in the 3rd sub array is 3, and the largest number in the 4th sub array is 4.