I have an array that I created using `Array(...)` and `Array.prototype.map`, like this:

```
var array_name = Array(255).map(function(undef, i) {
  return i + 1;
});

```

The values of this array are:

```
[1, 2, 3, ..., 253, 254, 255]

```

This is an array that won't get modified, so the first value of this array will always be `1` and the last value of this array will always be `255`.

I already know the index of each value is `{value} - 1`, so `200` would be `199`, `199` would be `198`, so on and so forth.

Let's say I want `255`'s opposite value, which would be `0`, I could get that using `array_name[0]`, but what if I wanted `200`'s opposite value, how would I know what the opposite index of `199` is so I could get it's value?