How to get the index of the min value in an array using JavaScript?

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 ...).

In this case it follows this way

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

is evaluated as

Math.max(11, 10, 10)

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.

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:

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

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