```typescript
export function mergeIntervals(intervals: number[][]): number[][] {
  if (!Array.isArray(intervals) || !intervals.every(interval => Array.isArray(interval) && interval.length === 2)) {
    throw new Error("Input must be an array of non-empty arrays with two numeric elements.");
  }

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

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

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

    if (currentInterval[1] >= nextInterval[0]) {
      // Intervals overlap or touch, merge them
      currentInterval[1] = Math.max(currentInterval[1], nextInterval[1]);
    } else {
      // No merge possible, add the current interval and update it to the next one
      merged.push(currentInterval);
      currentInterval = nextInterval;
    }
  }

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

  return merged;
}
```