Jest adds multiple layers on-top:

- A mock library (also called stubs) in order to create fake functions and assert that they are called as you want: http://facebook.github.io/jest/docs/mock-functions.html#cont...

- A way to provide an alternate implementation for dependencies using require(): http://facebook.github.io/jest/docs/common-js-testing.html#c...

- It inspects the shape of your dependencies and return a mocked version of them: http://facebook.github.io/jest/docs/automatic-mocking.html#c...

- Finally, it works in node instead of the browser and uses jsdom to let you test code related to the DOM. (It also runs tests in parallel)

jest is a test runner that includes a few additional things to try to make all aspects of writing and running tests a little easier...such as:

* Searches for tests to find + execute so that you can put tests near the modules they are testing

* Runs tests in parallel processes so that your test runs finish sooner

* Replace's node's built-in require() function with a custom one that acts almost entirely the same -- except (by default) it returns mocked versions of modules (rather than the real things) when your tests run

* Gives you a fake DOM implementation (using jsdom) so that you can test code that depends on DOM APIs on the command line without having to boot up a browser