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

  const sortedIntervals = intervals.slice().sort((a, b) => a[0] - b[0]);
  
  let merged: number[][] = [];
  let currentInterval = sortedIntervals[0];

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

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

// Edge case checks
console.log(mergeIntervals([])); // []
console.log(mergeIntervals([[5,5],[7,8]])); // [[5,5],[7,8]]
console.log(mergeIntervals([[10,20],[20,30]])); // [[10,30]]
console.log(() => mergeIntervals([[-1,-1]])); // throws Error
```