Return the item at the given index in the array, allowing for positive and negative integers. Negative integers count back from the last item in the array.

array_at.js

/**
 * Return the item at the given index in the array, allowing for positive and
 * negative integers. Negative integers count back from the last item
 * in the array.
 * 
 * {@link https://gist.github.com/kieranbarker/1bb4ae463f3cd8138b4805983de49f4d}
 * 
 * @param {Array} array The array
 * @param {Number} index The index
 * @returns {*} The item at the given index
 */
function array_at(array, index) {
  if (!Array.isArray(array)) {
    throw new TypeError('Expected first argument to be an array.');
  }

  if (typeof index !== 'number') {
    throw new TypeError('Expected second argument to be a number.');
  }

  return index >= 0 ? array[index] : array[array.length + index];
}

Read the blog post:

https://kbarker.dev/blog/the-array-at-method-in-vanilla-js/