tiny-pattern-ts/development/testing.md

# Testing

For this library the types _are_ the feature, so a runtime-only test loop would
verify the wrong thing. The commands are in
[CONTRIBUTING.md](../CONTRIBUTING.md); this file records why the loop is shaped
the way it is.

## Type-driven development

The rules — the loop and the pairing rule — are in
[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
What follows is why and what was rejected.

#### Decision (2026-09)

The compile-time expectation is written before the runtime assertion, and both
before the implementation.

#### Why

- A runtime-only test can pass while the type is wrong, so a type-level library
  would ship a broken feature its tests bless.
- The type error is a more precise spec than a failing assertion, because it
  states the exact expected type before the logic exists.

#### Rejected

- Runtime-first (classic red/green): it verifies the value, not the contract,
  and the contract is the product.
- Testing the type only: it would not catch handler dispatch or the `_`
  fallback (see `src/primitive.test.ts`).

The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
in
[CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
`c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
build step, and the runner relies on the `.ts` import-extension convention (see
[tooling.md](./tooling.md#source-imports-use-ts-extensions)).

## Handler arguments

#### Decision (2026-09)

A handler's `expectTypeOf(shape)` is always paired with an assertion on the
argument `dispatch` actually passed: `assert.equal` where the handler runs for
one shape, `assert.ok(s === … || s === …)` over the set a `_` fallback accepts
(`assert` is imported as `strict`, so each comparison is `Object.is`). Where a
test should also prove that the _exact_ value reached the handler unchanged,
the fallback returns the shape verbatim and the call site asserts it.

#### Why

- The parameter's type is what the compiler inferred from the pattern; the
  argument is what the runtime passed. Only the second can drift, and the keys
  whose property name differs from their value (`true`, `null`, `1`) are
  exactly where it can — see [library.md](./library.md).
- `String(shape)` at the call site keeps every type expectation and every
  return-value assertion green; the argument assertions fail (10 of the 33
  tests). Without them the suite never looks at the passed argument.
- Returning the shape verbatim costs a widening pattern nothing: the remainder
  type joins the union of handler returns in place of a marker literal, so the
  test still shows the widening it is named for.

#### Rejected

- A recorded `unknown[]` of every fallback call compared with `deepEqual`:
  strong, but it couples the assertion to call order, and the sink sits three
  blocks away from the value it observes.
- `typeof` checks: they cannot separate `2` from its key text `"2"`, which is
  the drift a fallback with a numeric remainder can hit.
- One expected value asserted inline in a fallback: its argument is a _set_ of
  shapes, so only the disjunction holds on every call.

## AAA ordering

The rule is in
[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).

#### Decision (2026-09)

Test bodies read arrange → act → assert: inputs (the factory) set up first, the
subject exercised once from them, all checks last — types then runtime. The
blocks are labeled with `// Arrange` / `// Act` / `// Assert` comments and
separated by a blank line; an empty block drops its label.

#### Why

- Interleaved setup/checks hide what runs vs. what is observed; the eye
  re-reads the block to find the seams.
- A factory built mid-test invites a second throwaway call of the subject;
  arranging it once makes the positive construction and the negative
  `Parameters<…>` check share one source of truth.
- Labels make the seams explicit, not inferred — grep-able and reviewable
  without reading the statements.

#### Rejected

- Unlabeled ordering (bare blank lines): the seams still have to be found by
  reading; the labels cost nothing.

## Test helpers

The rule is enforced by `import/no-relative-parent-imports`; this section records
why the mechanism is shaped like this.

#### Decision (2026-09)

A shared test helper — code that scattered `*.test.ts` files import to do their
testing — lives under `src/util/__tests__/` and is addressed by the
`#test-utils/…` self-reference (`package.json#imports`:
`"#test-utils/*": "./src/util/__tests__/*"`), never by a relative path:

```ts
import { LspSession } from "#test-utils/lsp-completion.ts";
```

#### Why

- The lint rule bans upward (`../`) imports, and a cross-cutting helper can
  always be placed above _some_ scattered consumer, wherever it goes. Name
  beats path: a `#test-utils/…` specifier has no direction, so the rule never
  fires and file moves only touch the one mapping in `package.json`.
- `#…` is Node's reserved prefix for _private_ subpath imports: publishing
  `package.json` leaks nothing and resolves nothing for consumers.
- `__tests__` as the folder name is not about tests living there; it is the
  directory pattern `tsconfig.build.json` already excludes, so a helper can
  never be emitted into `dist/` and shipped by accident.
- Node's own resolver handles `#…` under `--strip-types`, and `tsc` resolves it
  via the same `imports` field — one mechanism for runtime and type gate, no
  loader needed.
- The helper uses `node:` builtins (it drives a language server), so
  `import/no-nodejs-modules` is off for `src/util/__tests__/**` in
  `.oxlintrc.json` — the same exception the `scripts/**` scope carries.
- Helpers carry no library coupling: their tests probe in-memory documents
  whose contextual types are written inline, so only the helper's own contract
  (marker handling, position, label extraction) is under test. A test that
  asserts a _matcher's_ popup belongs with the matcher.

#### Rejected

- A helper beside the tests (`src/util/test.ts`, flat `src/`): composes only
  while `src/` stays flat; the first nested test reaching it reintroduces the
  banned upward import.
- Bare `~/…` specifier: not valid in `imports` (keys must start with `#`) —
  resolution fails at runtime with `ERR_MODULE_NOT_FOUND`. A `#~/…` “home”
  shorthand was dropped in review; `#test-utils/…` states what it is.
- tsconfig `paths` alias: resolves for `tsc` but not for plain
  `node --test --strip-types` (no loader hook), breaking the fast tier.
- Turning `import/no-relative-parent-imports` off for test files: the rule
  still guards non-test helpers importing each other, and the exemption is
  only needed for the one specifier the mapping already solves cleanly.

## Autocomplete

#### Decision (2026-09)

Completion is verified by driving the repo's own language server
(`tsc --lsp --stdio`, the same server pi's LSP extension talks to) through
the test helper `#test-utils/lsp-completion.ts`
(`src/util/__tests__/lsp-completion.ts`), not through the type system:

```sh
node --strip-types src/util/__tests__/lsp-completion.ts <file> [<marker>]
```

The script prints the labels the server offers at a `/*COMPLETE*/` marker inside
`<file>` (the marker is stripped before the document is sent). Its `LspSession`
is imported by `src/util/__tests__/lsp-completion.test.ts` — which tests the
helper itself against inline documents, never the library's code — and by
`src/primitive.test.ts`, where the same probe asserts the matcher's popup;
the CLI is for manual inspection.

#### Why

- Completion is a contextual-type property: it depends on which overload
  signature TypeScript picks for the object literal, and no type-level assertion
  observes that.
- `Parameters<typeof factory>[0]` resolves only the _last_ overload, so it is
  not the popup's contextual type either — see
  [library.md § Matcher shape](./library.md#matcher-shape).
- The server is the only ground truth; the script reproduces what the editor
  shows.

#### Rejected

- **`expect-type` would not work**: there is no operator for “the popup offers
  these labels”. `toExtend` / `toEqualTypeOf` test assignability and cannot say
  which overload supplied the contextual type.
- **Checking by hand in the editor**: not reproducible in review or by an agent.
- **`@ts-expect-error` at a completion position**: it asserts the absence of a
  compile error, not the presence of specific labels.

#### Known issue

- Each test spawns its own `tsc` server so the tests share no state and pass in
  any order; the file is an integration test (~1.6 s) that needs `node_modules`.
  `didOpen` is handled in order before the completion request, so no settle
  delay is needed.
- The server answers some requests with a string id (`client/registerCapability`);
  the client must tolerate `string | number` ids or the server stalls.

## Known issues

- The type-aware linter misidentifies `expectTypeOf()` as a floating promise.
  It is a known false positive, so `typescript/no-floating-promises` is off for
  `**/*.test.ts` in the `.oxlintrc.json` override rather than repeated as a
  file-level header (see
  [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).