Updated: Your browser might silently change setTimeout(f, 0) to setTimeout(f, 4)

Changes

When you're nesting `setTimeout(f, 0)` calls, your browser will silently increase the delay to 5 milliseconds after the fourth level of nesting. This is called "timeout clamping".
Deeply nested `setTimeout` can happen when you're using a non-native `Promise` polyfill with long chains of `then` calls.
The native `Promise` implementation ([Can I use link](http://caniuse.com/#feat=promises)) uses [microtasks](https://makandracards.com/makandra/40978-tasks-microtasks-queues-and-schedules-jakearchibald-com) and is thus not subject to timeout clamping.
If you need short nested timeouts, you can use an [setImmediate](https://github.com/YuzuJS/setImmediate) polyfill. This employs various hacks to give you access to the microtask queue.
Measuring timeout clamping
--------------------------
Here is a deeply nested `setTimeout` call:
```js
var start = Date.now();
function printElapsed() {
  var now = Date.now();
  console.log(now - start);
}
setTimeout(function() {
  printElapsed();
  setTimeout(function() {
    printElapsed();
    setTimeout(function() {
      printElapsed();
      setTimeout(function() {
        printElapsed();
        setTimeout(function() {
          printElapsed();
          setTimeout(function() {
            printElapsed();
            setTimeout(function() {
              printElapsed();
              setTimeout(function() {
                printElapsed();
                setTimeout(function() {
                  printElapsed();
                  setTimeout(function() {
                    printElapsed();
                    setTimeout(function() {
                      printElapsed();
                    }, 0);
                  }, 0);
                }, 0);
              }, 0);
            }, 0);
          }, 0);
        }, 0);
      }, 0);
    }, 0);
  }, 0);
}, 0);
```
This yields the following messages:
```raw
2
3
5
6
11
16
20
25
29
33
38
```
-Observe how after `6` the time difference jumps from 2 to 5 milliseconds.
+Observe how after `6` the time difference jumps from 2 to 5 milliseconds.
+
+
+
+## Timeouts are also clamped in background tabs
+
+On a similar note, all major browsers have implemented throttling rules for `setInterval` and `setTimeout` calls from tabs that are currently in the background. You can test it yourself by running this little test suite and change to a different tab during its runtime. The expected runtime drastically increases in Chrome, Firefox and Safari:
+<https://testbed.nicon.nl/timeouttest/>
+
+See also: [Stop animations and network polling when the document tab isn't visible](https://makandracards.com/makandra/41736-stop-animations-network-polling-document-tab-visible)
+
+