Jest Setup Example

This project experiments with an alternative to beforeEach when testing. This wraps it, it.only, and it.skip to allow for 3 arguments:

1. The title of your test
2. An object or function that provide data to your test (more on that later)
3. The test itself that receives data as its argument

it('accepts a setup function', generateMockUser, ({ user }) => {
  // test your mock user here just like normal
});

Background

For me, beforeEach works fine for setup code that is very side effect oriented. These are "fire and forget" functions such as stubbing out the network or resetting a mock. I think beforeEach starts to break down when we want to produce data that our tests will consume. The pattern I've seen most results in a laundry list of let declarations that are (re)assigned as part of the beforeEach callback.

let testUserId;

beforeEach(() => {
  testUserId = 123;
});

With only a few items this isn't so bad, but as a test file starts to grow this can become hard to follow and maintain.

Proposal

What would be nice is to have a function produce data that is fed to our test. This has a few benefits:

1. We can use standard function composition to deal with complex setups.
2. There's no longer a need for let variables since data is given to your test as local variables
3. Setup functions can easily be moved out of a test because they don't interact with local scope
4. We can also define a teardown function associated to the setup to bundle them together.

This repo is a Proof of Concept to see what testing in this manner would look like.

Test Examples

There are a few examples:

• ./src/basics.test.js gives an overview of using this approach
• ./src/enzymeExamplePureSetup.test.js gives a more involved example using enzyme
• ./src/enzymeExamplePureSetupExternal.test.js shows how easy it is to move all of the setup outside of the test (if you wanted to do that).
• ./src/enzymeExampleClassic.test.js shows the same example using beforeEach for comparison purposes. Obviously, there would be numerous ways to structure the example, but I tried to model it after patterns I have seen often in actual code.

You can run the tests via yarn test.

Implementation

The actual implementation is in ./src/setupTests.js and relies on wrapping the existing jest methods. I actually think this could work to some extent, but the intention of this repo is just to prove out the testing pattern as a whole. If the pattern seems viable then I think it would be awesome to see if Jest would consider supporting this first class in the code.