jest-canvas-mock

What is jest-canvas-mock?

The jest-canvas-mock npm package is a mock for the HTML Canvas API, designed to be used with the Jest testing framework. It allows developers to write tests for canvas operations without the need for a browser environment. This is particularly useful for unit testing canvas-related code in Node.js environments or in continuous integration systems where a DOM may not be available.

What are jest-canvas-mock's main functionalities?

Mocking Canvas Context

This feature allows you to mock the 2D context of a canvas element, enabling you to test canvas drawing methods like fillRect.

import 'jest-canvas-mock';
test('mock canvas context', () => {
  const canvas = document.createElement('canvas');
  const ctx = canvas.getContext('2d');
  ctx.fillRect(0, 0, 100, 100);
  expect(ctx.fillRect).toHaveBeenCalledWith(0, 0, 100, 100);
});

Mocking Canvas Image Data

This feature allows you to mock the creation of image data, which is useful for testing image manipulation on the canvas.

import 'jest-canvas-mock';
test('mock canvas image data', () => {
  const canvas = document.createElement('canvas');
  const ctx = canvas.getContext('2d');
  const imageData = ctx.createImageData(100, 100);
  expect(imageData.data.length).toBe(40000);
});

Mocking Canvas Path Operations

This feature allows you to mock canvas path operations like beginPath, moveTo, lineTo, and stroke, which is useful for testing drawing paths on the canvas.

import 'jest-canvas-mock';
test('mock canvas path operations', () => {
  const canvas = document.createElement('canvas');
  const ctx = canvas.getContext('2d');
  ctx.beginPath();
  ctx.moveTo(50, 50);
  ctx.lineTo(100, 100);
  ctx.stroke();
  expect(ctx.beginPath).toHaveBeenCalled();
  expect(ctx.moveTo).toHaveBeenCalledWith(50, 50);
  expect(ctx.lineTo).toHaveBeenCalledWith(100, 100);
  expect(ctx.stroke).toHaveBeenCalled();
});

Other packages similar to jest-canvas-mock

canvas-prebuilt

canvas-prebuilt is a prebuilt version of the 'canvas' package that provides a native canvas implementation for Node.js. It is similar to jest-canvas-mock in that it allows for canvas API testing, but it differs in that it provides an actual implementation rather than a mock, which may be more suitable for integration tests or when a real canvas environment is required.

README

jest-canvas-mock

Mock canvas when run unit test cases with jest. For more browser environment, you can use jest-electron for real browser runtime.

Install

This should only be installed as a development dependency (devDependencies) as it is only designed for testing.

npm i --save-dev jest-canvas-mock

Setup

In your package.json under the jest, create a setupFiles array and add jest-canvas-mock to the array.

{
  "jest": {
    "setupFiles": ["jest-canvas-mock"]
  }
}

If you already have a setupFiles attribute you can also append jest-canvas-mock to the array.

{
  "jest": {
    "setupFiles": ["./__setups__/other.js", "jest-canvas-mock"]
  }
}

More about in configuration section.

Setup file

Alternatively you can create a new setup file which then requires this module or add the require statement to an existing setup file.

__setups__/canvas.js

import 'jest-canvas-mock';
// or
require('jest-canvas-mock');

Add that file to your setupFiles array:

"jest": {
  "setupFiles": [
    "./__setups__/canvas.js"
  ]
}

Reset

If you reset the jest mocks (for example, with jest.resetAllMocks()), you can call setupJestCanvasMock() to re-create it.

import { setupJestCanvasMock } from 'jest-canvas-mock';

beforeEach(() => {
  jest.resetAllMocks();
  setupJestCanvasMock();
});

Mock Strategy

This mock strategy implements all the canvas functions and actually verifies the parameters. If a known condition would cause the browser to throw a TypeError or a DOMException, it emulates the error. For instance, the CanvasRenderingContext2D#arc function will throw a TypeError if the radius is negative, or if it was not provided with enough parameters.

// arc throws a TypeError when the argument length is less than 5
expect(() => ctx.arc(1, 2, 3, 4)).toThrow(TypeError);

// when radius is negative, arc throws a dom exception when all parameters are finite
expect(() => ctx.arc(0, 0, -10, 0, Math.PI * 2)).toThrow(DOMException);

The function will do Number type coercion and verify the inputs exactly like the browser does. So this is valid input.

expect(() => ctx.arc('10', '10', '20', '0', '6.14')).not.toThrow();

Another part of the strategy is to validate input types. When using the CanvasRenderingContext2D#fill function, if you pass it an invalid fillRule it will throw a TypeError just like the browser does.

expect(() => ctx.fill('invalid!')).toThrow(TypeError);
expect(() => ctx.fill(new Path2D(), 'invalid!')).toThrow(TypeError);

We try to follow the ECMAScript specification as closely as possible.

Snapshots

There are multiple ways to validate canvas state using snapshots. There are currently three methods attached to the CanvasRenderingContext2D class. The first way to use this feature is by using the __getEvents method.

/**
 * In order to see which functions and properties were used for the test, you can use `__getEvents`
 * to gather this information.
 */
const events = ctx.__getEvents();

expect(events).toMatchSnapshot(); // jest will assert the events match the snapshot

The second way is to inspect the current path associated with the context.

ctx.beginPath();
ctx.arc(1, 2, 3, 4, 5);
ctx.moveTo(6, 7);
ctx.rect(6, 7, 8, 9);
ctx.closePath();

/**
 * Any method that modifies the current path (and subpath) will be pushed to an event array. When
 * using the `__getPath` method, that array will sliced and usable for snapshots.
 */
const path = ctx.__getPath();
expect(path).toMatchSnapshot();

The third way is to inspect all of the successful draw calls submitted to the context.

ctx.drawImage(img, 0, 0);

/**
 * Every drawImage, fill, stroke, fillText, or strokeText function call will be logged in an event
 * array. This method will return those events here for inspection.
 */
const calls = ctx.__getDrawCalls();
expect(calls).toMatchSnapshot();

In some cases it may be useful to clear the events or draw calls that have already been logged.

// Clear events
ctx.__clearEvents();

// Clear draw calls
ctx.__clearDrawCalls();

Finally, it's possible to inspect the clipping region calls by using the __getClippingRegion function.

const clippingRegion = ctx.__getClippingRegion();
expect(clippingRegion).toMatchSnapshot();

The clipping region cannot be cleared because it's based on the stack values and when the .clip() function is called.

Override default mock return value

You can override the default mock return value in your test to suit your need. For example, to override return value of toDataURL:

canvas.toDataURL.mockReturnValueOnce(
  'data:image/png;base64, iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg=='
);

Contributors

• @hustcc
• @jtenner
• @evanoc0
• @lekha
• @yonatankra
• @LitoMore
• @hrd543
• @danielrentz

License

MIT@hustcc.