```typescript
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!Array.isArray(intervals) || intervals.length === 0) {
    throw new Error('Input must be a non-empty array of [start, end] pairs');
  }

  // Sort intervals by start
  intervals.sort((a, b) => a[0] - b[0]);

  const merged: number[][] = [];
  let currentInterval: number[] = intervals[0];

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

    if (currentInterval[1] >= nextInterval[0]) {
      // Merge the intervals
      currentInterval[1] = Math.max(currentInterval[1], nextInterval[1]);
    } else {
      // Add the merged interval to the result
      merged.push(currentInterval);
      currentInterval = nextInterval;
    }
  }

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

  return merged;
}
```