Is there a jest config that will fail tests on console.warn? - jestjs

Question

How do I configure jest tests to fail on warnings?
console.warn('stuff');
// fail test

Answer 1

You can use this simple override :
let error = console.error
console.error = function (message) {
error.apply(console, arguments) // keep default behaviour
throw (message instanceof Error ? message : new Error(message))
}
You can make it available across all tests using Jest setupFiles.
In package.json :
"jest": {
"setupFiles": [
"./tests/jest.overrides.js"
]
}
Then put the snippet into jest.overrides.js

Answer 2

For those using create-react-app, not wanting to run npm run eject, you can add the following code to ./src/setupTests.js:
global.console.warn = (message) => {
throw message
}
global.console.error = (message) => {
throw message
}
Now, jest will fail when messages are passed to console.warn or console.error.
create-react-app Docs - Initializing Test Environment

Answer 3

I implemented this recently using jest.spyOn introduced in v19.0.0 to mock the warn method of console (which is accesses via the global context / object).
Can then expect that the mocked warn was not called, as shown below.
describe('A function that does something', () => {
it('Should not trigger a warning', () => {
var warn = jest.spyOn(global.console, 'warn');
// Do something that may trigger warning via `console.warn`
doSomething();
// ... i.e.
console.warn('stuff');
// Check that warn was not called (fail on warning)
expect(warn).not.toHaveBeenCalled();
// Cleanup
warn.mockReset();
warn.mockRestore();
});
});

Answer 4

There is a useful npm package that helps you to achieve that: jest-fail-on-console
It's easily configurable.
Install:
npm i -D jest-fail-on-console
Configure:
In a file used in the setupFilesAfterEnv option of Jest, add this code:
import failOnConsole from 'jest-fail-on-console'
failOnConsole()
// or with options:
failOnConsole({ shouldFailOnWarn: false })

Answer 5

I decided to post a full example based on user1823021 answer
describe('#perform', () => {
var api
// the global.fetch is set to jest.fn() object globally
global.fetch = jest.fn()
var warn = jest.spyOn(global.console, 'warn');
beforeEach(function() {
// before every test, all mocks need to be resetted
api = new Api()
global.fetch.mockReset()
warn.mockReset()
});
it('triggers an console.warn if fetch fails', function() {
// In this test fetch mock throws an error
global.fetch.mockImplementationOnce(() => {
throw 'error triggered'
})
// I run the test
api.perform()
// I verify that the warn spy has been triggered
expect(warn).toHaveBeenCalledTimes(1);
expect(warn).toBeCalledWith("api call failed with error: ", "error triggered")
});
it('calls fetch function', function() {
// I create 2 more mock objects to verify the fetch parameters
const url = jest.fn()
const config = jest.fn()
api.url = url
api.config = config
// I run the test
api.perform()
// I verify that fetch has been called with url and config mocks
expect(global.fetch).toHaveBeenCalledTimes(1)
expect(global.fetch).toBeCalledWith(url, config)
expect(warn).toHaveBeenCalledTimes(0)
});
})
the #perform method I am testing
class Api {
constructor(auth) {
this._credentials = auth
}
perform = async () => {
try {
return await fetch(this.url, this.config)
} catch(error) {
console.warn('api call failed with error: ', error)
}
}
}

Answer 6

You can set the environment variable CI=true before running jest which will cause it to fail tests on warnings in addition to errors.
Example which runs all test files in the test folder:
CI=true jest ./test
Automated CI/CD pipelines such as Github Actions set CI to true by default, which can be one reason why a unit test will pass on your local machine when warnings are thrown, but fail in the pipeline.
(Here is the Github Actions documentation on default environment variables: https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables)