```ts
export function mergeIntervals(intervals: number[][]): number[][] {
    if (intervals.some(interval => interval[0] > interval[1])) {
        throw new Error("Invalid interval: start greater than end");
    }

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

    for (const interval of sortedIntervals) {
        if (!merged.length || merged[merged.length - 1][1] < interval[0]) {
            merged.push(interval);
        } else {
            merged[merged.length - 1][1] = Math.max(merged[merged.length - 1][1], interval[1]);
        }
    }

    return merged;
}
```