Add testing/fixture docs

[andyrichardson]

About

Docs on how to test + create fixtures with Urql!

It covers

• Testing calls to the client
• Forcing states (fetching, response, error, etc)
• Handling multiple hooks
• Simulating multiple responses

View markdown

Notes

• This is intended to cover all the info required to get testing (not a deep dive on how Wonka streams work)
• The examples use Enzyme mounts but principles apply to any library (including those with generic names)
• @bstro read my mind - Fix How to write tests with Provider #550

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[JoviDeCroock]

This looks very good to me! Great work

[wgolledge]

Looks awesome. I'll be reading this more thoroughly myself 😅

[bstro]

I'm attempting to follow these instructions right now and am finding that <Provider> does not typecheck when I give it the following client object:

const mockClient = {
  executeQuery: () => {},
  executeMutation: () => {},
  executeSubscription: () => {},
}

I am calling Provider like so:

    <Provider value={mockClient}>
      …
    </Provider>

and the typescript error is:

Type '{ executeQuery: () => void; executeMutation: () => void; executeSubscription: () => void; }'
 is missing the following properties from type 'Client':
  url, exchange, suspense, preferGetMethod, and 15 more.  TS2740

[andyrichardson]

@bstro this is because the mocks in the example don't match the actual client.

The way to go is to cast this as an any. I'll add a note about this to the docs.

const mockClient: any = {
  executeQuery: () => {},
  executeMutation: () => {},
  executeSubscription: () => {},
}