test(testing): improve FakeTime testing (#5123)

testing/time_test.ts

@@ -1,6 +1,7 @@
 // Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
 import {
   assert,
+  assertAlmostEquals,
   assertEquals,
   assertInstanceOf,
   assertMatch,
@@ -644,3 +645,80 @@ Deno.test("new FakeTime() throws if the time is already faked", () => {
   using _time = new FakeTime();
   assertThrows(() => new FakeTime());
 });
+
+Deno.test("Faked timer functions throws when called after FakeTime is restored", () => {
+  let fakeSetTimeout: typeof setTimeout;
+  let fakeClearTimeout: typeof clearTimeout;
+  let fakeSetInterval: typeof setInterval;
+  let fakeClearInterval: typeof clearInterval;
+  {
+    using _time: FakeTime = new FakeTime();
+    fakeSetTimeout = setTimeout;
+    fakeClearTimeout = clearTimeout;
+    fakeSetInterval = setInterval;
+    fakeClearInterval = clearInterval;
+  }
+  assertThrows(() => fakeSetTimeout(() => {}, 0), TimeError, "no fake time");
+  assertThrows(() => fakeClearTimeout(0), TimeError, "no fake time");
+  assertThrows(() => fakeSetInterval(() => {}, 0), TimeError, "no fake time");
+  assertThrows(() => fakeClearInterval(0), TimeError, "no fake time");
+});
+
+Deno.test("Faked Date.now returns real time after FakeTime is restored", () => {
+  let fakeDateNow: typeof Date.now;
+  {
+    using _time: FakeTime = new FakeTime();
+    fakeDateNow = Date.now;
+  }
+  assertAlmostEquals(Date.now(), fakeDateNow());
+});
+
+Deno.test("FakeTime can be constructed with number, Date, or string", () => {
+  {
+    using _time = new FakeTime(1000);
+    assertEquals(Date.now(), 1000);
+  }
+
+  {
+    using _time = new FakeTime(new Date(2000));
+    assertEquals(Date.now(), 2000);
+  }
+
+  {
+    using _time = new FakeTime("Thu Jan 01 1970 00:00:03 GMT+0000");
+    assertEquals(Date.now(), 3000);
+  }
+});
+
+Deno.test("FakeTime throws when NaN is provided", () => {
+  assertThrows(() => new FakeTime(NaN), TimeError, "invalid start");
+});
+
+Deno.test("FakeTime.restore() throws when the time is already restored", () => {
+  const _time = new FakeTime();
+  FakeTime.restore();
+  assertThrows(() => FakeTime.restore(), TimeError, "time already restored");
+});
+
+Deno.test("time.restore() throws when the time is already restored", () => {
+  const time = new FakeTime();
+  time.restore();
+  assertThrows(() => time.restore(), TimeError, "time already restored");
+});
+
+Deno.test("time.now = N throws when N < time.now", () => {
+  using time = new FakeTime(1000);
+  assertThrows(
+    () => {
+      time.now = 999;
+    },
+    Error,
+    "time cannot go backwards",
+  );
+});
+
+Deno.test("time.start returns the started time of the fake time", () => {
+  using time = new FakeTime(1000);
+  time.now = 2000;
+  assertEquals(time.start, 1000);
+});