```typescript
/** 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) {
    let bucket = groups.get(word.length);
    if (bucket === undefined) {
      bucket = [word];
      groups.set(word.length, bucket);
    } else {
      bucket.push(word);
    }
  }
  return groups;
}
```