jest-canvas-mock

by hustcc

:last_quarter_moon: A module used to mock canvas in Jest.

155 Stars 20 Forks Last release: 24 days ago (v2.3.0) 86 Commits 7 Releases

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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.

Build Status Coverage Status npm npm Mentioned in Awesome Jest

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"
  ]
}

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

License

[email protected]hustcc.

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