There are multiple ways.

1. Using Math max function

let array = [-1, 10, 30, 45, 5, 6, 89, 17];
console.log(Math.max(...array))

2. Using reduce

let array = [-1, 10, 30, 45, 5, 6, 89, 17];
console.log(array.reduce((element,max) => element > max ? element : max, 0));

3. Implement our own function

let array = [-1, 10, 30, 45, 5, 6, 89, 17];

function getMaxOutOfAnArray(array) {
  let maxNumber = -Infinity;
  array.forEach(number => { maxNumber =  number > maxNumber ? number :  maxNumber; })
  console.log(maxNumber);
}

getMaxOutOfAnArray(array);

Copyvar array = [3, 6, 2, 56, 32, 5, 89, 32];
var largest = 0;

for (let i=0; i < array.length; i++) {
    if (array[i] > largest) {
        largest = array[i];
    }
}

console.log(largest);

If you NEED to use a loop, then u shouldn't use Math.max() as it takes an undefined list of parameters (not an array, and no other types than numbers). You can still manage to filter the array beforehand or check the type.

The error you do is that you use the function Math.max() in a loop that already "works as a loop"

Here is the spec :

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/max

A simple way to do a loop is :

max = 0
for(item in array2){
    if(item>max){
        max=item
    }
}
console.log(max)

edit: you can check with typeof to avoid errors and initiate the value of max with -Infinity or else you can't get the max out of negative numbers

var array = [3, 4, 5, 21.15, 21, 9];
var largest = 0;

    for (i=0; i<array.length; i++) {
    if (array[i]>largest) {
        largest = array[i];
    }
}
console.log(largest);

Iteration 1:
Check if array[0] i.e. 3 is greater than 0 => True : largest = 3

Iteration 2:
Check if array[1] i.e. 4 is greater than 3 => True : largest = 4

Iteration 3:
Check if array[2] i.e. 5 is greater than 4 => True : largest = 5

Iteration 4:
Check if array[3] i.e. 21.25 is greater than 5 => True : largest = 21.25

Iteration 5:
Check if array[4] i.e. 21 is greater than 21.25 => False: largest = 21.25

Iteration 6:
Check if array[5] i.e. 9 is greater than 21.25 => False: largest = 21.25

To cover negative numbers in array, it is safe to assign largest to be the first element in array.

var arr = [2,3,4];
var largest = arr[0];
 
arr.forEach(function(elem){
  if(largest < elem) 
  largest = elem;
});
console.log(largest);