at.js

type AtRange = number | [number, number];

const at = <T>(arr: T[], ...ranges: AtRange[]): (T | undefined)[] =>
  ranges.flatMap((range) => {
    if (typeof range === "number") {
      return range >= 0 ? arr[range] : arr[arr.length + range - 1];
    }

    return range[0] >= 0
      ? arr.slice(range[0], range[1] + 1)
      : arr.slice(arr.length + range[0] - 1, arr.length + range[1]);
  });

console.log(at("hello world".split(""), 1, [-3, 0], -10));