Why is math.max() returning NaN on an array of integers?

The reason why your code doesn't work is because Math.max is expecting each parameter to be a valid number. This is indicated in the documentation as follows:

If at least one of arguments cannot be converted to a number, the result is NaN.

In your instance you are only providing 1 argument, and that 1 value is an array not a number (it doesn't go as far as checking what is in an array, it just stops at knowing it isn't a valid number).

One possible solution is to explicitly call the function by passing an array of arguments. Like so:

Math.max.apply(Math, data);

What this effectively does is the same as if you manually specified each argument without an array:

Math.max(4, 2, 6, 1, 3, 7, 5, 3);

And as you can see, each argument is now a valid number, so it will work as expected.

Spreading an array

You can also spread the array. This essentially treats the array as if each item is being passed as it's own argument.

Math.max(...data);

if you see doc for Math.max you can see next description

Because max() is a static method of Math, you always use it as Math.max(), rather than as a method of a Math object you created (Math is not a constructor).

If no arguments are given, the result is -Infinity.

If at least one of arguments cannot be converted to a number, the result is NaN.

When you call Math.max with array parameter like

Math.max([1,2,3])

you call this function with one parameter - [1,2,3] and javascript try convert it to number and get ("1,2,3" -> NaN) fail.
So result as expected - NaN

NOTE: if array with just one number - all work correctly

 Math.max([23]) // return 23

because [23] -> "23" -> 23 and covert to Number is done.

If you want get max element from array you should use apply function, like

Math.max.apply(Math,[1,2,3])

or you can use the new spread operator

Math.max(...[1,2,3])

It's not working because you are passing an array as the parameter instead of comma separated numbers. Try spreading the array like this:

data = [4, 2, 6, 1, 3, 7, 5, 3];    alert(Math.max(...data));