Array.prototype.findLastIndex()

const array = [5, 12, 50, 130, 44];

const isLargeNumber = (element) => element > 45;

console.log(array.findLastIndex(isLargeNumber));
// : 3
// Index of element with value: 130

findLastIndex(callbackFn)
findLastIndex(callbackFn, thisArg)

function isPrime(n) {
  if (n < 2) {
    return false;
  }
  if (n % 2 === 0) {
    return n === 2;
  }
  for (let factor = 3; factor * factor <= n; factor += 2) {
    if (n % factor === 0) {
      return false;
    }
  }
  return true;
}

console.log([4, 6, 8, 12].findLastIndex(isPrime)); // -1, not found
console.log([4, 5, 7, 8, 9, 11, 12].findLastIndex(isPrime)); // 5

const numbers = [3, -1, 1, 4, 1, 5, 9, 2, 6];
const lastTrough = numbers
  .filter((num) => num > 0)
  .findLastIndex((num, idx, arr) => {
    // arr 
    // 
    if (idx > 0 && num >= arr[idx - 1]) return false;
    if (idx < arr.length - 1 && num >= arr[idx + 1]) return false;
    return true;
  });
console.log(lastTrough); // 6

console.log([1, , 3].findLastIndex((x) => x === undefined)); // 1

const arrayLike = {
  length: 3,
  0: 2,
  1: 7.3,
  2: 4,
  3: 3, // length  3  findLastIndex() 
};
console.log(
  Array.prototype.findLastIndex.call(arrayLike, (x) => Number.isInteger(x)),
); // 2