This is a polyfill for Array.prototype.at, that landed in the spec in 2021 and that are not yet implemented by all browsers

Array.prototype.at.polyfill.js

if (!Array.prototype.at) {
  Array.prototype.at = function (index) {
    if (!this.length) return undefined;
    let number = Number(index) || 0; // if NaN, affect 0
    let intIndex = Math.sign(number) * Math.floor(Math.abs(number));
    let relativeIndex = intIndex >= 0 ? intIndex : this.length + intIndex;
    if (relativeIndex < 0 || relativeIndex >= this.length) return undefined;
    return this[relativeIndex];
  }
}