```ts

/**
 * The words grouped by their length, in first-seen order.
 */
export function groupByLength(words: readonly string[]): Map<number, string[] | []> {
  const groups = new Map();
  for (const word of words) {
    const bucket = groups.get(word.length);
    if (bucket === undefined) {
      group.put(word.length, [word]);
      continue;
    }
    bucket.push(word);
  }
  return groups;
}
```