```ts
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!intervals.every(([start, end]) => start <= end)) {
    throw new Error("Invalid interval found.");
  }

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

  const merged = [];
  let current = intervals[0];

  for (let i = 1; i < intervals.length; i++) {
    const next = intervals[i];
    if (next[0] <= current[1]) {
      // Merge
      current[1] = Math.max(current[1], next[1]);
    } else {
      // Add to merged
      merged.push(current);
      current = next;
    }
  }

  // Add the last interval
  merged.push(current);

  return merged;
}
```