@minoris/jest-dom-mocks

Jest mocking utilities for working with the DOM.

Installation

$ yarn add @minoris/jest-dom-mocks

Setup

This package provides two methods that should be included in the jest setup files:

• ensureMocksReset
• installMockStorage

ensureMocksReset

Should be called in the beforeEach method of the jest each-test setup file. For example:

import {ensureMocksReset} from '@minoris/jest-dom-mocks';

beforeEach(() => {
  ensureMocksReset();
});

this will ensure that appropriate error messages are shown if a DOM object is mocked without beign restored for the next test.

installMockStorage

Should be called in the jest setup file. For example:

import {installMockStorage} from '@minoris/jest-dom-mocks';

installMockStorage();

this will install the localStorage and sessionStorage mocks onto the global window object.

Example Usage

In this example, we are testing a NumberTransitioner component using Jest and Enzyme. Note that parts of this file have been omitted in order to focus in on the relevant parts of the example.

import {clock, animationFrame} from '@minoris/jest-dom-mocks';

it('transitions to the next number after being updated', () => {
  clock.mock();
  animationFrame.mock();

  const duration = 1000;
  const rendered = mount(
    <NumberTransitioner duration={duration}>{100}</NumberTransitioner>,
  );
  rendered.setProps({children: 200});

  clock.tick(duration / 4);
  animationFrame.runFrame();
  expect(rendered.text()).toBe('125');

  clock.tick(duration / 2);
  animationFrame.runFrame();
  expect(rendered.text()).toBe('175');

  clock.restore();
  animationFrame.restore();
});

API Reference

The mocks provided can be divided into 3 primary categories:

• standard mocks
• fetch mock
• storage mocks

Standard Mocks

The following standard mocks are available:

• animationFrame
• clock
• location
• matchMedia
• timer

Each of the standard mocks can be installed, for a given test, using standardMock.mock(), and must be restored before the end of the test using standardMock.restore().

For example:

import {location} from '@minoris/jest-dom-mocks';

beforeEach(() => {
  location.mock();
});

afterEach(() => {
  location.restore();
});

it('does a thing', () => {
  // run test code here
});

Or, if you just need to mock something for a single test:

import {location} from '@minoris/jest-dom-mocks';

it('does a thing', () => {
  location.mock();

  // run test code here

  location.restore();
});

Some of the standard mocks include additional features:

AnimationFrame.runFrame(): void

Executes all queued animation callbacks.

Clock.mock(now: number | Date): void

In addition to the usual .mock() functionality (with no arguments), the Clock object can be mocked by passing in a number or Date object to use as the current system time.

Clock.tick(time: number): void

Ticks the mocked Clock ahead by time milliseconds.

Clock.setTime(time: number): void

Sets the system time to the given time.

MatchMedia.mock(media?: MediaMatching): void

In addition to the usual .mock() functionality (with no arguments), the MatchMedia object can be mocked by passing in a MediaMatching function to use as the implementation.

The MediaMatching function has the following interface:

interface MediaMatching {
  (mediaQuery: string): Partial<MediaQueryList>;
}

it takes a mediaQuery string as input and returns a partial MediaQueryList to use as the result of window.matchMedia(mediaQuery). The partial result will be merged with the default values:

{
  media: '',
  addListener: noop,
  removeListener: noop,
  matches: false
}

MatchMedia.setMedia(media?: MediaMatching): void

Sets the implementation function for the mocked MatchMedia object. see above (MatchMedia.mock(media?: MediaMatching): void) for details on how MediaMatching works.

You can also call setMedia with no arguments to restore the default implementation.

Timer.runAllTimers(): void

Runs all system timers to completion.

Timer.runTimersToTime(time: number): void

Runs all system timers to the given time.

Fetch Mock

We use a version of fetch-mock that is augmented to ensure that it is properly unmocked after each test run. See the API of fetch-mock for more details.

Storage mock

The storage mocks are a bit different than the other mocks, because they serve primarily as a polyfill for the localStorage and sessionStorage APIs. The following standard API methods are implemented:

• getItem
• setItem
• removeItrem
• clear

Each of these are wrapped in a jest spy, which is automatically restored at the end of the test run.