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

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

  let merged: number[][] = [];
  for (let i = 0; i < sortedIntervals.length; i++) {
    const [start, end] = sortedIntervals[i];
    if (end < start) throw new Error('Invalid interval: start is greater than end');

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

  return merged;
}
```