 ```ts
export function mergeIntervals(intervals: number[][]): number[][] {
    if (!Array.isArray(intervals) || intervals.some(interval => interval.length !== 2)) {
        throw new Error("Input must be an array of exactly two-element arrays.");
    }

    if (intervals.some(([start, end]) => start > end)) {
        throw new Error("Interval start cannot be greater than its end.");
    }

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

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

    for (let i = 1; i < intervals.length; i++) {
        const [start, end] = intervals[i];
        if (currentInterval[1] >= start) {
            currentInterval[1] = Math.max(currentInterval[1], end);
        } else {
            result.push([...currentInterval]);
            currentInterval = [start, end];
        }
    }

    result.push(currentInterval);
    return result;
}
```