jest-canvas-snapshot-serializer

Jest Snapshot Serializer to create comparable snapshots of canvas elements.

Usage

Make sure you're either running jest with environment set to jsdom (default setting) or you have setup jsdom in the test setup file.

Install canvas-prebuilt and this module

npm install --save-dev canvas-prebuilt jest-canvas-snapshot-serializer

and add it as serializer

// myTest.spec.js
import canvasSerializer from "jest-canvas-snapshot-serializer";

expect.addSnapshotSerializer(canvasSerializer);

test("my awesome test", () => {
    const canvas = document.createElement("canvas");

    // canvas must have a width and height attribute
    // otherwise there is no image to serialize
    canvas.setAttribute("width", "200");
    canvas.setAttribute("height", "200");

    drawAwesomeImage(canvas);

    expect(canvas).toMatchSnapshot();
});

Running the test creates a snapshot file like:

exports["my awesome test 1"] = `
<canvas
  width="200"
  height="200"
  data-snapshot-image="51e09c5637c8c4cf463ce0da78329bcca119..."
/>
`

The snapshot now contains a hashed representation of the drawn image. So we're informed about canvas image regressions \o/

Furthermore this serializer will create an image file next to the snapshot file.

.
├── __snapshots__
│   ├── myTest.spec.js.snap
│   ├── myTest.spec.js.snap.my-awesome-test.canvas-image.png
│   └── myTest.spec.js.snap.my-awesome-test.canvas-image.dirty.png
└── myTest.spec.js

There is also a dirty image file if jest is running without --updateSnapshot and the persisted snapshot doesn't match the current implementation. So you can compare the original/persisted image with the current one. The dirty image is deleted as soon as jest updates the snapshot.

FAQ

Why a snapshot serializer instead of a custom matcher like .toMatchCanvasSnapshot?

Using a custom matcher we'd have to implement the same stuff already provided by jest-snapshot (e.g. success and error feedback). Whereas using a serializer we only have to take care about the serialization of the canvas element and writing/deleting the image files.

License

Apache License 2.0