unit test raises error because of .getContext() is not implemented

jest-canvas-mock
jest test webgl
jest mock imagedata
npm install --save-dev canvas
unit testing canvas
cannot read property 'backingstorepixelratio' of null
jest/test-sequencer
jest tests

I am writing tests using Jest for components that use canvas elements. I keep getting an error when I run my tests that looks like this.

Error: Not implemented: HTMLCanvasElement.prototype.getContext (without installing the canvas npm package)

From my understanding Jest uses jsdom for its testing and that jsdom is compatible with canvas if you install the canvas or canvas-prebuilt packages.

I have tried installing each of these packages and neither of them have resolved the error. The only thing that I think could be going wrong is that jsdom cannot find the canvas or canvas-prebuilt packages. Does anyone know a way to fix this error or test to see if jsdom is finding the other packages? Thanks a lot!

You can create your own mock of the function in a jest setup script

HTMLCanvasElement.prototype.getContext = () => { 
  // return whatever getContext has to return
};

Error: Not implemented: HTMLCanvasElement.prototype.getContext , Using jest-canvas-mock still throw this error when running jest: Not getContext (without installing the canvas npm package) in CI as a base for our containers in which we run our unit and integration tests. So when we run the ci with jest-​canvas-mock@1.0.0, will execute print the error, because of the  The only workarounds I see are to create dedicated unit test wrappers for every component I want to test, or to wrap every live component in a data-getter component that just does getContext and passes the values in via props. Either approach is a lot of extra work for something that feels like it should be table-stakes (unit testing).

I had similar problem with my Jest tests, and solved it by installing jest-canvas-mock.

Evaluating Redux Saga Test Libraries, This functional approach makes Redux easy to test, but it means that your reducers select(getContext); const user = yield call(getUser, action.payload, context); Rather than test your saga in isolation, as a unit, integration testing libraries it('raises failed action', () => { const error = new Error("404 Not  Error: Not implemented: HTMLCanvasElement.prototype.getContext (without installing the canvas npm package) In my case it I had to mock this method in my .spec.ts file. HTMLCanvasElement.prototype.getContext = jest.fn(); However, in my scenario I tested particles.js, having Jest as a testing framework I still had another error

Both answers above work. However, just wanted to add that in case of create-react-app, it doesn't support the 'setupFiles' required by jest conf in package.json for jest-canvas-mock.

I got around this by adding the import statement in each of the test files that were invoking the canvas element(in my case 3). Then, I moved the import statement to the setupTests.js file and removed that requirement as well.

Testing constructs, Currently, TypeScript is the only supported language for testing AWS CDK you add will cause your snapshot test to fail even if existing features still work. Validation tests help you "fail fast" by making sure your AWS CDK constructs raise errors placeholders indicate existing parts of the file that should not be changed. Python testing framework provides the following assertion methods to check that exceptions are raised. assertRaises(exception, callable, *args, **kwds) Test that an exception (first argument) is raised when a function is called with any positional or keyword arguments.

jestjs, But the gitlab-ci pipeline fails, cause it cant . Issue with Jest during the unit tests - getContext() is not implemented them, this error was raised: SyntaxError: Cannot use import statement outside a module I thought it should be a problem . You can write your own assertWarns function to incapsulate catch_warnings context. I've just implemented it the following way, with a mixin: class WarningTestMixin(object): 'A test which checks if the specified warning was raised' def assertWarns(self, warning, callable, *args, **kwds): with warnings.catch_warnings(record=True) as warning_list: warnings.simplefilter('always') result = callable

26.3. unittest — Unit testing framework, The final block shows a simple way to run the tests. unittest.main() provides a MyTestCase) skipped 'not supported in this library version' test_nothing Any exception raised by this method will be considered an error rather than a test failure. instead of assertTrue(a == b)), because they provide a better error message  In NUnit, you can use: Assert.DoesNotThrow(<expression>); to assert that your code does not throw an exception. Although the test would fail if an exception is thrown even if there was no Assert around it, the value of this approach is that you can then distinguish between unmet expectations and bugs in your tests, and you have the option of adding a custom message that will be displayed in

Testing, You can also unit test an Angular app with other testing libraries and test runners. it('raises the selected event when clicked', () => { const comp = new This is actually a convenience method, implemented as fixture. That's not a problem when you run the CLI ng test command because it getContext('2d'); context. Unit test basics. 08/07/2019; 14 minutes to read +12; In this article. Check that your code is working as expected by creating and running unit tests. It's called unit testing because you break down the functionality of your program into discrete testable behaviors that you can test as individual units.

Comments
  • This worked perfectly. I had tried to do something similar to this but it turns out I didn't include the path to the file in my setupFiles array. Thank you so much for your help!
  • No worries! Feel free to up vote and mark my answer with the checkmark if it resolved your problem.
  • Add this setup to your setupFile.js
  • I'm also using create-react-app and didn't want to eject. I imported import 'jest-canvas-mock' in my setupTest.js alone, and so far it has seemed to work.