@sinonjs/fake-timers


JavaScript implementation of the timer
APIs; setTimeout, clearTimeout, setImmediate, clearImmediate, setInterval, clearInterval, requestAnimationFrame, cancelAnimationFrame, requestIdleCallback,
and cancelIdleCallback, along with a clock instance that controls the flow of time. FakeTimers also provides a Date
implementation that gets its time from the clock.

In addition in browser environment @sinonjs/fake-timers provides a performance implementation that gets its time
from the clock. In Node environments FakeTimers provides a nextTick implementation that is synchronized with the
clock - and a process.hrtime shim that works with the clock.

@sinonjs/fake-timers can be used to simulate passing time in automated tests and other
situations where you want the scheduling semantics, but don't want to actually
wait.

@sinonjs/fake-timers is extracted from Sinon.JS and targets
the same runtimes.

Autocomplete, IntelliSense and TypeScript definitions

Version 7 introduced JSDoc to the codebase. This should provide autocomplete and type suggestions in supporting IDEs. If
you need more elaborate type support, TypeScript definitions for the Sinon projects are independently maintained by the
Definitely Types community:

``
npm install -D @types/sinonjs__fake-timers
`

Installation

@sinonjs/fake-timers can be used in both Node and browser environments. Installation is as easy as

`sh
npm install @sinonjs/fake-timers
`

If you want to use @sinonjs/fake-timers in a browser you can either build your own bundle or
use Skypack.

Usage

To use @sinonjs/fake-timers, create a new clock, schedule events on it using the timer
functions and pass time using the tick method.

`js
// In the browser distribution, a global FakeTimers is already available
var FakeTimers = require("@sinonjs/fake-timers");
var clock = FakeTimers.createClock();

clock.setTimeout(function () {
console.log(
"The poblano is a mild chili pepper originating in the state of Puebla, Mexico.",
);
}, 15);

// ...

clock.tick(15);
`

Upon executing the last line, an interesting fact about the
Poblano will be printed synchronously to
the screen. If you want to simulate asynchronous behavior, please see the async function variants (
eg clock.tick(time) vs await clock.tickAsync(time)).

The next, runAll, runToFrame, and runToLast methods are available to advance the clock. See the
API Reference for more details.

$3

When using @sinonjs/fake-timers to test timers, you will most likely want to replace the native
timers such that calling setTimeout actually schedules a callback with your
clock instance, not the browser's internals.

Calling install with no arguments achieves this. You can call uninstall
later to restore things as they were again.
Note that in NodeJS the timers
and timers/promises modules will also receive fake timers when
using global scope.

`js
// In the browser distribution, a global FakeTimers is already available
var FakeTimers = require("@sinonjs/fake-timers");

var clock = FakeTimers.install();
// Equivalent to
// var clock = FakeTimers.install(typeof global !== "undefined" ? global : window);

setTimeout(fn, 15); // Schedules with clock.setTimeout

clock.uninstall();
// setTimeout is restored to the native implementation
`

To hijack timers in another context pass it to the install method.

`js
var FakeTimers = require("@sinonjs/fake-timers");
var context = {
setTimeout: setTimeout, // By default context.setTimeout uses the global setTimeout
};
var clock = FakeTimers.withGlobal(context).install();

context.setTimeout(fn, 15); // Schedules with clock.setTimeout

clock.uninstall();
// context.setTimeout is restored to the original implementation
`

Usually you want to install the timers onto the global object, so call install
without arguments.

#### Automatically incrementing mocked time

FakeTimers supports the possibility to attach the faked timers to any change
in the real system time. This means that there is no need to tick() the
clock in a situation where you won't know when to call tick().

Please note that this is achieved using the original setImmediate() API at a certain
configurable interval config.advanceTimeDelta (default: 20ms). Meaning time would
be incremented every 20ms, not in real time.

An example would be:

`js
var FakeTimers = require("@sinonjs/fake-timers");
var clock = FakeTimers.install({
shouldAdvanceTime: true,
advanceTimeDelta: 40,
});

setTimeout(() => {
console.log("this just timed out"); //executed after 40ms
}, 30);

setImmediate(() => {
console.log("not so immediate"); //executed after 40ms
});

setTimeout(() => {
console.log("this timed out after"); //executed after 80ms
clock.uninstall();
}, 50);
`

In addition to the above, mocked time can be configured to advance more quickly
using clock.setTickMode({ mode: "nextAsync" });. With this mode, the clock
advances to the first scheduled timer and fires it, in a loop. Between each timer,
it will also break the event loop, allowing any scheduled promise
callbacks to execute _before_ running the next one.

API Reference

$3

Creates a clock. The default
epoch is 0.

The now argument may be a number (in milliseconds) or a Date object.

The loopLimit argument sets the maximum number of timers that will be run when calling runAll() before assuming that
we have an infinite loop and throwing an error. The default is 1000.

$3

Installs FakeTimers using the specified config (otherwise with epoch 0 on the global scope).
Note that in NodeJS the timers
and timers/promises modules will also receive fake timers when
using global scope.
The following configuration options are available

| Parameter | Type | Default | Description |
| -------------------------------- | ----------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| config.now | Number/Date | 0 | installs FakeTimers with the specified unix epoch |
| config.toFake | String[] | ["setTimeout", "clearTimeout", "setImmediate", "clearImmediate","setInterval", "clearInterval", "Date", "requestAnimationFrame", "cancelAnimationFrame", "requestIdleCallback", "cancelIdleCallback", "hrtime", "performance"] | an array with explicit function names (or objects, in the case of "performance") to hijack. \_When not set, FakeTimers will automatically fake all methods e.g., FakeTimers.install({ toFake: ["setTimeout","nextTick"]}) will fake only setTimeout and nextTick |
| config.loopLimit | Number | 1000 | the maximum number of timers that will be run when calling runAll() |
| config.shouldAdvanceTime | Boolean | false | tells FakeTimers to increment mocked time automatically based on the real system time shift (e.g. the mocked time will be incremented by 20ms for every 20ms change in the real system time) |
| config.advanceTimeDelta | Number | 20 | relevant only when using with shouldAdvanceTime: true. increment mocked time by advanceTimeDelta ms every advanceTimeDelta ms change in the real system time. |
| config.shouldClearNativeTimers | Boolean | false | tells FakeTimers to clear 'native' (i.e. not fake) timers by delegating to their respective handlers. These are not cleared by default, leading to potentially unexpected behavior if timers existed prior to installing FakeTimers. |
| config.ignoreMissingTimers | Boolean | false | tells FakeTimers to ignore missing timers that might not exist in the given environment |

$3

Allows configuring how the clock advances time, automatically or manually.

There are 3 different types of modes for advancing timers:

- {mode: 'manual'}: Timers do not advance without explicit, manual calls to the tick
APIs (clock.nextAsync, clock.runAllAsync, etc). This mode is equivalent to false.
- {mode: 'nextAsync'}: The clock will continuously break the event loop, then run the next timer until the mode changes.
As a result, tests can be written in a way that is independent from whether fake timers are installed.
Tests can always be written to wait for timers to resolve, even when using fake timers.
- {mode: 'interval', delta?: }: This is the same as specifying shouldAdvanceTime: true with an advanceTimeDelta. If the delta is
not specified, 20 will be used by default.

The 'nextAsync' mode differs from interval in two key ways:

1. The microtask queue is allowed to empty between each timer execution,
as would be the case without fake timers installed.
1. It advances as quickly and as far as necessary. If the next timer in
the queue is at 1000ms, it will advance 1000ms immediately whereas interval,
without manually advancing time in the test, would take 1000 / advanceTimeDelta
real time to reach and execute the timer.

$3

Schedules the callback to be fired once timeout milliseconds have ticked by.

In Node.js setTimeout returns a timer object. FakeTimers will do the same, however
its ref() and unref() methods have no effect.

In browsers a timer ID is returned.

$3

Clears the timer given the ID or timer object, as long as it was created using
setTimeout.

$3

Schedules the callback to be fired every time timeout milliseconds have ticked
by.

In Node.js setInterval returns a timer object. FakeTimers will do the same, however
its ref() and unref() methods have no effect.

In browsers a timer ID is returned.

$3

Clears the timer given the ID or timer object, as long as it was created using
setInterval.

$3

Schedules the callback to be fired once 0 milliseconds have ticked by. Note
that you'll still have to call clock.tick() for the callback to fire. If
called during a tick the callback won't fire until 1 millisecond has ticked
by.

In Node.js setImmediate returns a timer object. FakeTimers will do the same,
however its ref() and unref() methods have no effect.

In browsers a timer ID is returned.

$3

Clears the timer given the ID or timer object, as long as it was created using
setImmediate.

$3

Schedules the callback to be fired on the next animation frame, which runs every
16 ticks. Returns an id which can be used to cancel the callback. This is
available in both browser & node environments.

$3

Cancels the callback scheduled by the provided id.

$3

Queued the callback to be fired during idle periods to perform background and low priority work on the main event loop.
Callbacks which have a timeout option will be fired no later than time in milliseconds. Returns an id which can be
used to cancel the callback.

$3

Cancels the callback scheduled by the provided id.

$3

Returns the number of waiting timers. This can be used to assert that a test
finishes without leaking any timers.

$3

Only available in Node.js, mimicks process.hrtime().

$3

Only available in Node.js, mimics process.nextTick to enable completely synchronous testing flows.

$3

Only available in browser environments, mimicks performance.now().

$3

Advance the clock, firing callbacks if necessary. time may be the number of
milliseconds to advance the clock by or a human-readable string. Valid string
formats are "08" for eight seconds, "01:00" for one minute and "02:34:10"
for two hours, 34 minutes and ten seconds.

The tickAsync() will also break the event loop, allowing any scheduled promise
callbacks to execute _before_ running the timers.

$3

Advances the clock to the the moment of the first scheduled timer, firing it.

The nextAsync() will also break the event loop, allowing any scheduled promise
callbacks to execute _before_ running the timers.

$3

Advance the clock by jumping forward in time, firing callbacks at most once.
time takes the same formats as clock.tick.

This can be used to simulate the JS engine (such as a browser) being put to sleep and resumed later, skipping
intermediary timers.

$3

Removes all timers and ticks without firing them, and sets now to config.now
that was provided to FakeTimers.install or to 0 if config.now was not provided.
Useful to reset the state of the clock without having to uninstall and install it.

$3

This runs all pending timers until there are none remaining. If new timers are added while it is executing they will be
run as well.

This makes it easier to run asynchronous tests to completion without worrying about the number of timers they use, or
the delays in those timers.

It runs a maximum of loopLimit times after which it assumes there is an infinite loop of timers and throws an error.

The runAllAsync() will also break the event loop, allowing any scheduled promise
callbacks to execute _before_ running the timers.

$3

This runs all pending microtasks scheduled with nextTick but none of the timers and is mostly useful for libraries
using FakeTimers underneath and for running nextTick items without any timers.

$3

Advances the clock to the next frame, firing all scheduled animation frame callbacks,
if any, for that frame as well as any other timers scheduled along the way.

$3

This takes note of the last scheduled timer when it is run, and advances the
clock to that time firing callbacks as necessary.

If new timers are added while it is executing they will be run only if they
would occur before this time.

This is useful when you want to run a test to completion, but the test recursively
sets timers that would cause runAll to trigger an infinite loop warning.

The runToLastAsync() will also break the event loop, allowing any scheduled promise
callbacks to execute _before_ running the timers.

$3

This simulates a user changing the system clock while your program is running.
It affects the current time but it does not in itself cause e.g. timers to fire;
they will fire exactly as they would have done without the call to
setSystemTime().

$3

Restores the original methods of the native timers or the methods on the object
that was passed to FakeTimers.withGlobal

$3

Implements the Date object but using the clock to provide the correct time.

$3

Implements the now method of the Performance
object but using the clock to provide the correct time. Only available in environments that support the Performance
object (browsers mostly).

$3

In order to support creating clocks based on separate or sandboxed environments (such as JSDOM), FakeTimers exports a
factory method which takes single argument global, which it inspects to figure out what to mock and what features to
support. When invoking this function with a global, you will get back an object with timers, createClock
and install - same as the regular FakeTimers exports only based on the passed in global instead of the global
environment.

Promises and fake time

If you use a Promise library like Bluebird, note that you should either call clock.runMicrotasks() or make sure to
_not_ mock nextTick.

Running tests

FakeTimers has a comprehensive test suite. If you're thinking of contributing bug
fixes or suggesting new features, you need to make sure you have not broken any
tests. You are also expected to add tests for any new behavior.

$3

`sh
npm test
`

Or, if you prefer more verbose output:

`
$(npm bin)/mocha ./test/fake-timers-test.js
`

$3

Mochify is used to run the tests in headless
Chrome.

`sh
npm test-headless
``

License

BSD 3-clause "New" or "Revised" License (see LICENSE file)