https://twitter.com/uhyo_/status/1283618353110753281

offsetRange.js

function offsetRange(arr) {
  return (
    arr.map(v => [v, v+1])
    .join(",")
    .replace(/(,\d+)\1/g, "-")
    .replace(/(-+,)\d+/g, (_, s) => "-" + (s.length))
    .replace(/(,\d+),\d+/g, (_, s) => `${s}-1`)
    .split(",")
    .map((v) => {
      const [offset, range] = v.split("-");
      return ({offset, range})
    })
  );
}

console.log(offsetRange([1, 2, 3, 5, 6, 9, 10]));