How do browsers pause/change Javascript when tab or window is not active?

Question

Background: I'm doing some user interface tests that need to detect if people are paying attention or not. But, this question is not about the page visibility API. Specifically, I would like to know how my Javascript code will be affected if the current tab is not active, or the browser window is not active, in different browsers. I've dug up the following so far: - ios 5 pauses javascript when tab is not active - `setInterval` and `setTimeout` delay is reduced when tabs are not active - seems like this just started appearing recently and can mess up Jasmine unit tests, around other things. - `requestAnimationFrame` is slowed when tab is not active (reasonable, can't think of why this would affect anyone too much) I have the following questions: - Other than mobile browsers, do desktop browsers ever pause JS execution when a tab is not active? When and which browsers? - Which browsers reduce the `setInterval` repeat? Is it just reduced to a limit or by a percentage? For example, if I…

Accepted Answer

Test One

I have written a test specifically for this purpose: Frame Rate Distribution: setInterval vs requestAnimationFrame

Note: This test is quite CPU intensive. `requestAnimationFrame` is not supported by IE 9- and Opera 12-.

The test logs the actual time it takes for a `setInterval` and `requestAnimationFrame` to run in different browsers, and gives you the results in the form of a distribution. You can change the number of milliseconds for `setInterval` to see how it runs under different settings. `setTimeout` works similarly to a `setInterval` with respect to delays. `requestAnimationFrame` generally defaults to the 60fps depending on the browser. To see what happens when you switch to a different tab or have an inactive window, simply open the page, switch to a different tab and wait for a while. It will continue to log the actual time it takes for these functions in an inactive tab.

Test Two

Another way to test it is to log the timestamp repeatedly with `setInterval` and `requestAnimationFrame` and view it in a detached console. You can see how frequently it is updated (or if it is ever updated) when you make the tab or window inactive.

- Test for `setInterval`

- Test for `requestAnimationFrame`

Results

Chrome Chrome limits the minimum interval of `setInterval` to around 1000ms when the tab is inactive. If the interval is higher than 1000ms, it will run at the specified interval. It does not matter if the window is out of focus, the interval is limited only when you switch to a different tab. `requestAnimationFrame` is paused when the tab is inactive.

```
// Provides control over the minimum timer interval for background tabs.
const double kBackgroundTabTimerInterval = 1.0;
```

https://codereview.chromium.org/6546021/patch/1001/2001

Firefox Similar to Chrome, Firefox limits the minimum interval of `setInterval` to around 1000ms when the tab (not the window) is inactive. However, `requestAnimationFrame` runs exponentially slower when the tab is inactive, with each frame taking 1s, 2s, 4s, 8s and so on.

```
// The default shortest interval/timeout we permit
#define DEFAULT_MIN_TIMEOUT_VALUE 4 // 4ms
#define DEFAULT_MIN_BACKGROUND_TIMEOUT_VALUE 1000 // 1000ms
```

https://hg.mozilla.org/releases/mozilla-release/file/0bf1cadfb004/dom/base/nsGlobalWindow.cpp#l296

Internet Explorer IE does not limit the delay in `setInterval` when the tab is inactive, but it pauses `requestAnimationFrame` in inactive tabs. It does not matter whether the window is out of focus or not.

Edge Starting from Edge 14, `setInterval` is capped at 1000ms in inactive tabs. `requestAnimationFrame` is always paused in inactive tabs.

Safari Just like Chrome, Safari caps `setInterval` at 1000ms when the tab is inactive. `requestAnimationFrame` is paused as well.

Opera Since the adoption of the Webkit engine, Opera exhibits the same behavior as Chrome. `setInterval` is capped at 1000ms and `requestAnimationFrame` is paused when the tab is inactive.

Summary

Repeating intervals for inactive tabs:

```
           setInterval     requestAnimationFrame
Chrome
9-         not affected    not supported
10         not affected    paused
11+        >=1000ms        paused

Firefox
3-         not affected    not supported
4          not affected    1s
5+         >=1000ms        2ns (n = number of frames since inactivity)

IE
9-         not affected    not supported
10+        not affected    paused

Edge
13-        not affected    paused
14+        >=1000ms        paused

Safari
5-         not affected    not supported
6          not affected    paused
7+         >=1000ms        paused

Opera
12-        not affected    not supported
15+        >=1000ms        paused
```

How do browsers pause/change Javascript when tab or window is not active?

Solution

Problem

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