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

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

  const result: number[][] = [];
  let currentStart = intervals[0][0];
  let currentEnd = intervals[0][1];

  for (let i = 1; i < intervals.length; i++) {
    const interval = intervals[i];

    if (interval[0] <= currentEnd) {
      // Extend the current interval
      currentEnd = Math.max(currentEnd, interval[1]);
    } else {
      // Add the previous interval to the result
      result.push([currentStart, currentEnd]);

      // Start a new interval
      currentStart = interval[0];
      currentEnd = interval[1];
    }
  }

  // Add the last interval to the result
  result.push([currentStart, currentEnd]);

  return result;
}
```