Additional tests

test/once.test.js

@@ -4,6 +4,11 @@ const {once} = require('./_30s.js');
 test('once is a Function', () => {
   expect(once).toBeInstanceOf(Function);
 });
-test('once is a Function', () => {
+test('once returns Function', () => {
   expect(typeof once(x => 10)).toBe('function');
 });
+test('once returns the result only once', () => {
+  let onced = once(x => x);
+  expect(onced(10)).toBe(10);
+  expect(onced(10)).toBe(undefined);
+});