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

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

Setup

In your

package.json

jest

setupFiles

jest-canvas-mock

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

If you already have a

setupFiles

jest-canvas-mock

{
  "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

__setups__/canvas.js

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

Add that file to your

setupFiles

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

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

DOMException

CanvasRenderingContext2D#arc

TypeError

// 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

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

fillRule

TypeError

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

__getEvents

/**
 * 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

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()

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

License

[email protected]hustcc.