```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.some(([start, end]) => start > end)) {
    throw new Error("Start cannot be greater than end");
  }

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

  const merged: number[][] = [];
  for (const interval of intervals) {
    if (merged.length === 0 || merged[merged.length - 1][1] < interval[0]) {
      merged.push(interval);
    } else {
      merged[merged.length - 1][1] = Math.max(merged[merged.length - 1][1], interval[1]);
    }
  }

  return merged;
}
```