Updated the test system

test/collatz.test.js

@@ -0,0 +1,22 @@
+const expect = require('expect');
+const {collatz} = require('./_30s.js');
+
+test('collatz is a Function', () => {
+  expect(collatz).toBeInstanceOf(Function);
+});
+test('When n is even, divide by 2', () => {
+  expect(collatz(8)).toBe(4);
+});
+test('When n is odd, times by 3 and add 1', () => {
+  expect(collatz(9)).toBe(28);
+});
+test('Eventually reaches 1', () => {
+  let n = 9;
+  while (true) {
+    if (n === 1) {
+      expect(n).toBe(1);
+      break;
+    }
+    n = collatz(n);
+  }
+});