How do I test a jest console.log
If you want to check that console.log
received the right parameter (the one that you passed in) you should check mock
of your jest.fn()
.
You also have to invoke your log
function, otherwise console.log
is never invoked:
it('console.log the text "hello"', () => {
console.log = jest.fn();
log('hello');
// The first argument of the first call to the function was 'hello'
expect(console.log.mock.calls[0][0]).toBe('hello');
});
or
it('console.log the text "hello"', () => {
console.log = jest.fn();
log('hello');
// The first argument of the first call to the function was 'hello'
expect(console.log).toHaveBeenCalledWith('hello');
});
If you're going with this approach don't forget to restore the original value of console.log
.
Another option is to use jest.spyOn
(instead of replacing the console.log
it will create a proxy to it):
it('console.log the text "hello"', () => {
const logSpy = jest.spyOn(console, 'log');
console.log('hello');
expect(logSpy).toHaveBeenCalledWith('hello');
});
Read more here.
Or you could do it like this:
it('calls console.log with "hello"', () => {
const consoleSpy = jest.spyOn(console, 'log');
console.log('hello');
expect(consoleSpy).toHaveBeenCalledWith('hello');
});
Another option is to save off a reference to the original log, replace with a jest mock for each test, and restore after all the tests have finished. This has a slight benefit to not polluting the test output and still being able to use the original log method for debugging purposes.
describe("Some behavior that will log", () => {
const log = console.log; // save original console.log function
beforeEach(() => {
console.log = jest.fn(); // create a new mock function for each test
});
afterAll(() => {
console.log = log; // restore original console.log after all tests
});
test("no log", () => {
// TODO: test something that should not log
expect(console.log).not.toHaveBeenCalled();
});
test("some log", () => {
// TODO: execute something that should log
expect(console.log).toHaveBeenCalledWith(
expect.stringContaining("something")
);
const message = console.log.mock.calls[0][0]; // get actual log message
log(message); // actually log out what the mock was called with
});
});