Ideally when you write your tests, each test can be run by itself in isolation. And you should be able to run your tests in any order. (There is an option in Jest to run your tests in a random order to help enforce this).

But sometimes things from previous tests leak into other tests. They might set a global variable, leave some mock/spied on functions, or mutate data.

In this test challenge we have a simple todo app that we are testing. It uses window.localStorage in the browser to read/write some data.

And if you haven't guessed already - this is leaking into other tests.