25 lines
1.3 KiB
Markdown
25 lines
1.3 KiB
Markdown
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---
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name: Mocking fetch in api-server route tests
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description: How to mock upstream fetch without breaking the test client's own request to the in-process Express server
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---
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When testing an api-server route that itself calls `fetch` (e.g. provider model
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discovery / connection tests), the route tests spin up the real Express app via
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`app.listen(0)` and hit it with `fetch(`${baseUrl}/api/...`)`. The app runs in the
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SAME process as the test, so `vi.spyOn(globalThis, "fetch")` intercepts BOTH the
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test client's request to the server AND the route's upstream provider call.
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**Rule:** capture `const realFetch = globalThis.fetch.bind(globalThis)` BEFORE
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mocking, and in the mock implementation delegate any request whose URL starts with
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the test server's `baseUrl` back to `realFetch`. Only synthesize a response for the
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upstream (non-localhost) URL.
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**Also:** the spy records the localhost passthrough call too, so don't assert on
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`mock.calls[0]` — filter to the call whose URL does NOT start with `baseUrl` to find
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the real upstream request. Likewise `not.toHaveBeenCalled()` / `toHaveBeenCalledTimes(1)`
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must be expressed in terms of the filtered upstream calls, not the raw spy.
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**Why:** without the passthrough, the test client's request to the in-process server
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gets the synthetic upstream body (or the thrown error), so every test fails before the
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route logic runs.
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