```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]) {
      // Merge the intervals
      currentInterval[1] = Math.max(currentInterval[1], nextInterval[1]);
    } else {
      // No intersection, add the current interval to the merged list
      merged.push([...currentInterval]);
      currentInterval = nextInterval;
    }
  }

  // Add the last interval if it's not merged with another one
  if (merged.length === 0 || merged[merged.length - 1][1] < currentInterval[0]) {
    merged.push([...currentInterval]);
  }

  return merged;
}
```