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@jest/fake-timers

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@jest/fake-timers


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Package description

What is @jest/fake-timers?

@jest/fake-timers is a package that provides fake timer functions for Jest, a popular JavaScript testing framework. It allows you to control the passage of time in your tests, making it easier to test code that relies on timers, such as setTimeout, setInterval, and Date.

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

Mocking setTimeout and setInterval

This feature allows you to mock setTimeout and setInterval functions, enabling you to control the passage of time in your tests. The code sample demonstrates how to use jest.useFakeTimers() to mock the timers and jest.advanceTimersByTime() to fast-forward time.

const { jest } = require('@jest/globals');

jest.useFakeTimers();

setTimeout(() => {
  console.log('This will be logged after 1 second');
}, 1000);

jest.advanceTimersByTime(1000); // Fast-forward time by 1 second

// Output: 'This will be logged after 1 second'

Mocking Date

This feature allows you to mock the Date object, enabling you to control the current date and time in your tests. The code sample demonstrates how to use jest.useFakeTimers('modern') to mock the Date object and jest.setSystemTime() to set a specific date and time.

const { jest } = require('@jest/globals');

jest.useFakeTimers('modern');

const now = Date.now();
console.log(now); // Outputs the current timestamp

jest.setSystemTime(new Date('2020-01-01'));

const newNow = Date.now();
console.log(newNow); // Outputs the timestamp for 2020-01-01

Clearing timers

This feature allows you to clear mocked timers, ensuring that the associated callbacks are not executed. The code sample demonstrates how to use clearTimeout() to clear a setTimeout timer.

const { jest } = require('@jest/globals');

jest.useFakeTimers();

const timeoutId = setTimeout(() => {
  console.log('This will not be logged');
}, 1000);

clearTimeout(timeoutId);

jest.advanceTimersByTime(1000); // Fast-forward time by 1 second

// No output

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Package last updated on 20 Feb 2024

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