findDisappearedNumbers.js

/**
 * @param {number[]} nums
 * @return {number[]}
 */
function findDisappearedNumbers(nums) {
	const set    = new Set(nums),
	      result = [];

	for (let i = 1; i <= nums.length; i++) {
		if (!set.has(i)) {
			result.push(i);
		}
	}

	return result;
}

module.exports = findDisappearedNumbers;