```ts

export function mergeIntervals(intervals: number[][]): number[][] {
    if (intervals.some(([start, end]) => start > end)) {
        throw new Error("Interval start greater than end");
    }

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

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

    for (let i = 1; i < intervals.length; i++) {
        const current = intervals[i];
        const lastMerged = merged[merged.length - 1];

        if (current[0] <= lastMerged[1]) {
            // Merge intervals
            lastMerged[1] = Math.max(lastMerged[1], current[1]);
        } else {
            // Add new interval
            merged.push(current);
        }
    }

    return merged;
}
```