swashata/enzyme-to-json-bug

Demonstration of enzyme to json bug

Break the test

This demonstrates how a silly thing can break your tests. It uses enzyme and enzyme-to-json to implement snapshot testing with jest.

The component against which we are running tests, has a onClick handler. We test the handler by jest.fn() mock function. We simulate the click with enzyme and test to see if the mock was called.

But it also changes the values associated with jest.fn() and the snapshot gets broken.

Steps to take

Just clone this repo and yarn install followed by npx jest.

yarn install
npx jest

It should fail.

Reason?

The reason is simple. The jest.fn() is getting called before matching the snapshot. So the timestamp or basically the internal implementation and values of the jest.fn() changes everytime the tests run. Hence the snapshots will never match.

Infact, when snapshot testing, the implementation should be done in a way that internal values do not change over time.

If we try to snapshot test this object

const currentTime = Date.now();
expect(currentTime).toMatchSnapshot();

it will always fail, because currentTime changes everytime the tests are called.

Rather we can mock it

const currentTime = 1526900471489;
expect(currentTime).toMatchSnapshot();

and it will pass.

Weird thing with enzyme-to-json

For some weird reason the failing snapshots when being updated, keeps on adding to the list. I will open an issue.