```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (intervals.length === 0) return [];

  intervals.forEach(([start, end]) => {
    if (start > end) throw new Error("Interval start cannot be greater than end");
  });

  const sortedIntervals = intervals.slice().sort((a, b) => a[0] - b[0]);
  let merged: number[][] = [sortedIntervals[0]];

  for (let i = 1; i < sortedIntervals.length; i++) {
    const lastMerged = merged[merged.length - 1];
    const currentInterval = sortedIntervals[i];

    if (currentInterval[0] <= lastMerged[1]) {
      merged[merged.length - 1] = [lastMerged[0], Math.max(lastMerged[1], currentInterval[1])];
    } else {
      merged.push(currentInterval);
    }
  }

  return merged;
}
```