PromiseMerge

/**
 * @param {unknown} value
 * @returns {boolean}
 */
function isPlainObject(value) {
	// Accept only plain object literals (or objects with null prototype).
	if (value === null || typeof value !== 'object' || Array.isArray(value)) {
		return false;
	}

	const proto = Object.getPrototypeOf(value);
	return proto === Object.prototype || proto === null;
}

/**
 * @param {Promise<unknown>} promiseA
 * @param {Promise<unknown>} promiseB
 * @returns {Promise<unknown>}
 */
export default function promiseMerge(promiseA, promiseB) {
	// Wait for both promises; if either rejects, Promise.all propagates that rejection reason.
	return Promise.all([promiseA, promiseB]).then(([valueA, valueB]) => {
		// Same-type numbers: add.
		if (typeof valueA === 'number' && typeof valueB === 'number') {
			return valueA + valueB;
		}

		// Same-type strings: concatenate in input order.
		if (typeof valueA === 'string' && typeof valueB === 'string') {
			return valueA + valueB;
		}

		// Same-type arrays: combine into one array in input order.
		if (Array.isArray(valueA) && Array.isArray(valueB)) {
			return [...valueA, ...valueB];
		}

		// Same-type plain objects: shallow-merge with valueB overriding matching keys.
		if (isPlainObject(valueA) && isPlainObject(valueB)) {
			return {...valueA, ...valueB};
		}

		// Any other combination is unsupported by this utility.
		throw 'Unsupported data types';
	});
}