function addToArray(array, element, checkDuplicates) {
  checkDuplicates = checkDuplicates ?? false;
  if (checkDuplicates) {
    const index = array.indexOf(element);
    if (index > -1) {
      return index;
    }
  }

  array.push(element);
  return array.length - 1;
}
Adds an element to an array and returns the element's index.

@param {Array} array The array to add to.
@param {object} element The element to add.
@param {boolean} [checkDuplicates=false] When <code>true</code>, if a duplicate element is found its index is returned and <code>element</code> is not added to the array.

@private