Updated the test system

test/unfold.test.js

@@ -0,0 +1,10 @@
+const expect = require('expect');
+const {unfold} = require('./_30s.js');
+
+test('unfold is a Function', () => {
+  expect(unfold).toBeInstanceOf(Function);
+});
+var f = n => (n > 50 ? false : [-n, n + 10]);
+test('Works with a given function, producing an array', () => {
+  expect(unfold(f, 10)).toEqual([-10, -20, -30, -40, -50]);
+});