Question: I wrote this function, which returns `-1`.

```js
function sayHello() {
  let arrayEdificiosNiveis = [11,10,10];
  var indexMenor = arrayEdificiosNiveis.indexOf(Math.max(arrayEdificiosNiveis));
  console.log(indexMenor);
}
sayHello();
```

I expected 0, not -1. How do I solve this problem?

Answer: 
You need to spread the array for getting the maximum. Otherwise you get `NaN` as value (via a stringed array) and this is not in the array (and not searchable).

A spreaded array takes all elements as parameter for the function ([spread syntax `...`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax)).

In this case it follows this way

```
Math.max(...[11, 10, 10])

```

is evaluated as

```
Math.max(11, 10, 10)

```

```js
function sayHello() {
  arrayEdificiosNiveis = [11, 10, 10];
  var indexMenor = arrayEdificiosNiveis.indexOf(Math.max(...arrayEdificiosNiveis));

  console.log(indexMenor);
}

sayHello();
```

A single loop solution:

But why not use

```
v > a[r] ? i : r

```

(which feels more natural, sort of) instead of

```
v <= a[r] ? r : i

```

The problem is the first comparison with the first element at index zero. at this time, `r = -1` and the element is `a[r] = undefined`.

A comparison with `undefined` and a relational operator of `<`, `<=`, `>` or `>=` is always `false` and that would return the wrong index of `-1` instead of zero and this result does not change for any other element of the array.

```js
const
    getFirstIndexOfMaxValue = array =>
        array.reduce((r, v, i, a) => v <= a[r] ? r : i, -1);            

console.log(getFirstIndexOfMaxValue([]));
console.log(getFirstIndexOfMaxValue([11]));
console.log(getFirstIndexOfMaxValue([11, 10]));
console.log(getFirstIndexOfMaxValue([11, 10, 10]));
console.log(getFirstIndexOfMaxValue([10, 11, 10]));
console.log(getFirstIndexOfMaxValue([10, 11, 10, 11]));
```
The problem in your approach is the complete array you're passing on the function `Math.max`:

```js
const arrayEdificiosNiveis = [11, 10, 10],
  max = Math.max(...arrayEdificiosNiveis);

console.log(arrayEdificiosNiveis.findIndex(elem => elem === max));
```

If it's not possible to use `Spread syntax`, you can call the function `apply` which receives an array as the parameters of function `Math.max`

```js
function sayHello() {
  const arrayEdificiosNiveis = [11, 10, 10],
    max = Math.max.apply(null, arrayEdificiosNiveis),
    indexMenor = arrayEdificiosNiveis.indexOf(max);
  console.log(indexMenor);
}
sayHello();
```