```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.some(interval => interval[0] > interval[1])) {
    throw new Error('Invalid interval');
  }

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

  const merged: number[][] = [];
  let currentInterval = intervals[0];

  for (let i = 1; i < intervals.length; i++) {
    const nextInterval = intervals[i];
    if (nextInterval[0] > currentInterval[1]) {
      merged.push(currentInterval);
      currentInterval = nextInterval;
    } else {
      currentInterval[1] = Math.max(currentInterval[1], nextInterval[1]);
    }
  }

  merged.push(currentInterval);

  return merged;
}
```