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

  // Validate and sort intervals
  const sortedIntervals = intervals.slice().sort((a, b) => a[0] - b[0]);
  for (const interval of sortedIntervals) {
    if (!Array.isArray(interval) || interval.length !== 2 || interval[0] > interval[1]) {
      throw new Error("Invalid interval");
    }
  }

  const merged: number[][] = [];
  let currentInterval = [...sortedIntervals[0]];

  for (let i = 1; i < sortedIntervals.length; i++) {
    const nextInterval = sortedIntervals[i];
    if (currentInterval[1] >= nextInterval[0]) {
      // Merge intervals
      currentInterval[1] = Math.max(currentInterval[1], nextInterval[1]);
    } else {
      merged.push([...currentInterval]);
      currentInterval = [...nextInterval];
    }
  }

  merged.push([...currentInterval]);

  return merged;
}
```