```typescript
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!intervals.every(range => range.length === 2)) {
    throw new Error("Each interval must be a pair of two numbers.");
  }

  intervals.sort((a, b) => a[0] - b[0]);

  const merged = [intervals[0]];

  for (let i = 1; i < intervals.length; i++) {
    const [currentStart, currentEnd] = merged.at(-1)!;
    const [nextStart, nextEnd] = intervals[i];

    if (currentEnd >= nextStart) {
      merged.at(-1)![1] = Math.max(currentEnd, nextEnd);
    } else {
      merged.push(intervals[i]);
    }
  }

  return merged;
}
```