Roll your own Array.prototype.indexOf()

custom-js-array-indexOf.js

// The long way to Array.prototype.indexOf(), written while making an image slide show

const images = [...document.querySelectorAll('[data-js="carousel-img"]')];

const activeEl = images.filter((img) => img.classList.contains('active'))[0];

const activeElIndex = images.reduce((acc, img, i) => {
  return acc != undefined
    ? acc
    : img.classList.contains('active')
    ? i
    : undefined;
}, undefined);