Array.prototype.at

/**
 * @template T
 * @param {number} index
 * @returns {T | undefined}
 */
Array.prototype.myAt = function (index) {
	const length = this.length >>> 0;

	// Convert index to integer (truncate toward zero), similar to ToIntegerOrInfinity.
	const numericIndex = Number(index);
	let integerIndex;

	if (Number.isNaN(numericIndex) || numericIndex === 0) {
		integerIndex = 0;
	} else if (!Number.isFinite(numericIndex)) {
		integerIndex = numericIndex;
	} else {
		integerIndex = numericIndex < 0 ? Math.ceil(numericIndex) : Math.floor(numericIndex);
	}

	const normalizedIndex = integerIndex >= 0 ? integerIndex : length + integerIndex;

	if (normalizedIndex < 0 || normalizedIndex >= length) {
		return undefined;
	}

	return this[normalizedIndex];
};

/**
Example:

const nums = [10, 20, 30, 40];

nums.myAt(0);    // 10
nums.myAt(2);    // 30
nums.myAt(-1);   // 40
nums.myAt(-3);   // 20

nums.myAt(1.9);  // 20 (index is truncated toward zero)
nums.myAt(-1.2); // 40

nums.myAt(99);   // undefined
nums.myAt(-99);  // undefined
*/