[[development-unit-tests]]
== Unit testing frameworks

{kib} is doing unit testing using `Jest`.

[discrete]
== Jest
Jest tests are stored in the same directory as source code files with the `.test.{js,mjs,ts,tsx}` suffix.

Each plugin and package contains it's own `jest.config.js` file to define its root, and any overrides 
to the jest-preset provided by `@kbn/test`. When working on a single plugin or package, you will find 
it's more efficient to supply the Jest configuration file when running.

["source","shell"]
-----------
yarn jest --config src/plugins/discover/jest.config.js
-----------

[discrete]
==== Writing Jest Unit Tests

In order to write those tests there are two main things you need to be aware of.
The first one is the different between `jest.mock` and `jest.doMock` 
and the second one our `jest mocks file pattern`. As we are running `js` and `ts`
test files with `babel-jest` both techniques are needed
specially for the tests implemented on Typescript in order to benefit from the 
auto-inference types feature.

[discrete]
==== Jest.mock vs Jest.doMock

Both methods are essentially the same on their roots however the `jest.mock` 
calls will get hoisted to the top of the file and can only reference variables 
prefixed with `mock`. On the other hand, `jest.doMock` won't be hoisted and can 
reference pretty much any variable we want, however we have to assure those referenced 
variables are instantiated at the time we need them which lead us to the next 
section where we'll talk about our jest mock files pattern.

[discrete]
==== Jest Mock Files Pattern 

Specially on typescript it is pretty common to have in unit tests 
`jest.doMock` calls which reference for example imported types. Any error 
will thrown from doing that however the test will fail. The reason behind that
is because despite the `jest.doMock` isn't being hoisted by `babel-jest` the 
import with the types we are referencing will be hoisted to the top and at the 
time we'll call the function that variable would not be defined.

In order to prevent that we develop a protocol that should be followed:

- Each module could provide a standard mock in `mymodule.mock.ts` in case 
there are other tests that could benefit from using definitions here. 
This file would not have any `jest.mock` calls, just dummy objects.

- Each test defines its mocks in `mymodule.test.mocks.ts`. This file 
could import relevant mocks from the generalised module's mocks 
file `(*.mock.ts)` and call `jest.mock` for each of them. If there is 
any relevant dummy mock objects to generalise (and to be used by 
other tests), the dummy objects could be defined directly on this file.

- Each test would import its mocks from the test mocks 
file mymodule.test.mocks.ts. `mymodule.test.ts` has an import 
like: `import * as Mocks from './mymodule.test.mocks'`, 
`import { mockX } from './mymodule.test.mocks'` 
or just `import './mymodule.test.mocks'` if there isn't anything 
exported to be used.

[discrete]
[[debugging-unit-tests]]
=== Debugging Unit Tests

The standard `yarn test` task runs several sub tasks and can take
several minutes to complete, making debugging failures pretty painful.
In order to ease the pain specialized tasks provide alternate methods
for running the tests.

You could also add the `--debug` option so that `node` is run using
the `--debug-brk` flag. You’ll need to connect a remote debugger such
as https://github.com/node-inspector/node-inspector[`node-inspector`]
to proceed in this mode.

[discrete]
=== Unit Testing Plugins

Even when using 
https://github.com/elastic/kibana/tree/main/packages/kbn-plugin-generator[Kibana
plugin generator] we do not enforce a way for unit testing your plugin. Please setup and you use 
the tools of your choice. If the plugin will live inside the Kibana repo `Jest` must be used.