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Clock

Introduction​

Accurately simulating time-dependent behavior is essential for verifying the correctness of applications. Utilizing Clock functionality allows developers to manipulate and control time within tests, enabling the precise validation of features such as rendering time, timeouts, scheduled tasks without the delays and variability of real-time execution.

The Clock API provides the following methods to control time:

  • setFixedTime: Sets the fixed time for Date.now() and new Date().
  • install: initializes the clock and allows you to:
    • pauseAt: Pauses the time at a specific time.
    • fastForward: Fast forwards the time.
    • runFor: Runs the time for a specific duration.
    • resume: Resumes the time.
  • setSystemTime: Sets the current system time.

The recommended approach is to use setFixedTime to set the time to a specific value. If that doesn't work for your use case, you can use install which allows you to pause time later on, fast forward it, tick it, etc. setSystemTime is only recommended for advanced use cases.

note

page.clock overrides native global classes and functions related to time allowing them to be manually controlled:

  • Date
  • setTimeout
  • clearTimeout
  • setInterval
  • clearInterval
  • requestAnimationFrame
  • cancelAnimationFrame
  • requestIdleCallback
  • cancelIdleCallback
  • performance
  • Event.timeStamp

Test with predefined time​

Often you only need to fake Date.now while keeping the timers going. That way the time flows naturally, but Date.now always returns a fixed value.

<div id="current-time" data-testid="current-time"></div>
<script>
  const renderTime = () => {
    document.getElementById('current-time').textContent =
        new Date().toLocaleString();
  };
  setInterval(renderTime, 1000);
</script>

Consistent time and timers​

Sometimes your timers depend on Date.now and are confused when the Date.now value does not change over time. In this case, you can install the clock and fast forward to the time of interest when testing.

<div id="current-time" data-testid="current-time"></div>
<script>
  const renderTime = () => {
    document.getElementById('current-time').textContent =
        new Date().toLocaleString();
  };
  setInterval(renderTime, 1000);
</script>
# Initialize clock with some time before the test time and let the page load
# naturally. `Date.now` will progress as the timers fire.
page.clock.install(time=datetime.datetime(2024, 2, 2, 8, 0, 0))
page.goto("http://localhost:3333")

# Pretend that the user closed the laptop lid and opened it again at 10am.
# Pause the time once reached that point.
page.clock.pause_at(datetime.datetime(2024, 2, 2, 10, 0, 0))

# Assert the page state.
expect(page.get_by_test_id("current-time")).to_have_text("2/2/2024, 10:00:00 AM")

# Close the laptop lid again and open it at 10:30am.
page.clock.fast_forward("30:00")
expect(page.get_by_test_id("current-time")).to_have_text("2/2/2024, 10:30:00 AM")

Test inactivity monitoring​

Inactivity monitoring is a common feature in web applications that logs out users after a period of inactivity. Testing this feature can be tricky because you need to wait for a long time to see the effect. With the help of the clock, you can speed up time and test this feature quickly.

<div id="remaining-time" data-testid="remaining-time"></div>
<script>
  const endTime = Date.now() + 5 * 60_000;
  const renderTime = () => {
    const diffInSeconds = Math.round((endTime - Date.now()) / 1000);
    if (diffInSeconds <= 0) {
      document.getElementById('remaining-time').textContent =
        'You have been logged out due to inactivity.';
    } else {
      document.getElementById('remaining-time').textContent =
        `You will be logged out in ${diffInSeconds} seconds.`;
    }
    setTimeout(renderTime, 1000);
  };
  renderTime();
</script>
<button type="button">Interaction</button>
# Initial time does not matter for the test, so we can pick current time.
page.clock.install()
page.goto("http://localhost:3333")
# Interact with the page
page.get_by_role("button").click()

# Fast forward time 5 minutes as if the user did not do anything.
# Fast forward is like closing the laptop lid and opening it after 5 minutes.
# All the timers due will fire once immediately, as in the real browser.
page.clock.fast_forward("05:00")

# Check that the user was logged out automatically.
expect(page.get_by_text("You have been logged out due to inactivity.")).to_be_visible()

Tick through time manually, firing all the timers consistently​

In rare cases, you may want to tick through time manually, firing all timers and animation frames in the process to achieve a fine-grained control over the passage of time.

<div id="current-time" data-testid="current-time"></div>
<script>
  const renderTime = () => {
    document.getElementById('current-time').textContent =
        new Date().toLocaleString();
  };
  setInterval(renderTime, 1000);
</script>
# Initialize clock with a specific time, let the page load naturally.
page.clock.install(
    time=datetime.datetime(2024, 2, 2, 8, 0, 0, tzinfo=datetime.timezone.pst),
)
page.goto("http://localhost:3333")
locator = page.get_by_test_id("current-time")

# Pause the time flow, stop the timers, you now have manual control
# over the page time.
page.clock.pause_at(datetime.datetime(2024, 2, 2, 10, 0, 0))
expect(locator).to_have_text("2/2/2024, 10:00:00 AM")

# Tick through time manually, firing all timers in the process.
# In this case, time will be updated in the screen 2 times.
page.clock.run_for(2000)
expect(locator).to_have_text("2/2/2024, 10:00:02 AM")