@sinonjs/fake-timers

What is @sinonjs/fake-timers?

The @sinonjs/fake-timers package is a utility for testing time-dependent code. It allows developers to simulate the passing of time in a controlled manner, without actually waiting. This is particularly useful for unit tests where you want to test timeouts, intervals, or the current date without causing delays in your test suite.

What are @sinonjs/fake-timers's main functionalities?

Simulating setTimeout

This feature allows you to simulate the behavior of setTimeout. By advancing the clock, you can immediately trigger timeouts without waiting.

const { install } = require('@sinonjs/fake-timers');
const clock = install();
setTimeout(() => console.log('Timeout called'), 100);
clock.tick(100); // Simulates the passing of 100 milliseconds

Simulating setInterval

This feature enables the simulation of setInterval. You can advance the clock to trigger intervals as many times as needed without real time passing.

const { install } = require('@sinonjs/fake-timers');
const clock = install();
setInterval(() => console.log('Interval called'), 100);
clock.tick(300); // Simulates the passing of 300 milliseconds, triggering the interval 3 times

Simulating Date

This feature allows you to set the current date to a specific timestamp, enabling you to test code that depends on the current date without relying on the system's clock.

const { install } = require('@sinonjs/fake-timers');
const clock = install({ now: 1577836800000 });
console.log(new Date()); // Logs the fake current date set to the start of 2020

Other packages similar to @sinonjs/fake-timers

timekeeper

Timekeeper is a package for controlling time in JavaScript. It allows freezing, traveling to a specific date, and mocking dates and times. Compared to @sinonjs/fake-timers, it offers a simpler API but lacks the ability to simulate the ticking of time, making it less versatile for testing intervals and timeouts.

lolex

Lolex is a predecessor of @sinonjs/fake-timers and offers similar functionalities for controlling JavaScript timers and the system clock. It provides a robust set of features for manipulating time in tests. However, @sinonjs/fake-timers is recommended for new projects as it is the more actively maintained and updated version of Lolex.

README

@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

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.

// 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.

Faking the native timers

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.

// 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.

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:

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);

API Reference

var clock = FakeTimers.createClock([now[, loopLimit]])

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.

var clock = FakeTimers.install([config])

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

var id = clock.setTimeout(callback, timeout)

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.

clock.clearTimeout(id)

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

var id = clock.setInterval(callback, timeout)

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.

clock.clearInterval(id)

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

var id = clock.setImmediate(callback)

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.

clock.clearImmediate(id)

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

clock.requestAnimationFrame(callback)

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.

clock.cancelAnimationFrame(id)

Cancels the callback scheduled by the provided id.

clock.requestIdleCallback(callback[, timeout])

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.

clock.cancelIdleCallback(id)

Cancels the callback scheduled by the provided id.

clock.countTimers()

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

clock.hrtime(prevTime?)

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

clock.nextTick(callback)

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

clock.performance.now()

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

clock.tick(time) / await clock.tickAsync(time)

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.

clock.next() / await clock.nextAsync()

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.

clock.jump(time)

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.

clock.reset()

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.

clock.runAll() / await clock.runAllAsync()

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.

clock.runMicrotasks()

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.

clock.runToFrame()

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.

clock.runToLast() / await clock.runToLastAsync()

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.

clock.setSystemTime([now])

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().

clock.uninstall()

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

Date

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

Performance

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).

FakeTimers.withGlobal

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.

On node:

npm test

Or, if you prefer more verbose output:

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

In the browser

Mochify is used to run the tests in headless Chrome.

npm test-headless

License

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