Preview:
function findMissing(arr1, arr2) {
	let str = arr1.filter(i => !arr2.includes(i)).join('');
	return Number(str);
}


 Test.assertEquals(findMissing([1, 2, 3], [1, 3]), 2);
  Test.assertEquals(findMissing([6, 1, 3, 6, 8, 2], [3, 6, 6, 1, 2]), 8);
  Test.assertEquals(findMissing([7], []), 7);
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