Testing Strategy

mdx-formatter uses a multi-layered testing approach. The TypeScript test suite in test/ defines the formatting contract and runs all tests through the Rust napi engine. The Rust test suite in crates/ validates the engine independently.

TypeScript Test Suite

The tests in test/ exercise the public format() API, which calls the Rust engine via napi-rs.

Test Files

File	Focus	Description
`formatter.test.ts`	Core formatting	Heading spacing, list indentation, JSX handling, Japanese text
`mdx-formatter.test.ts`	Advanced formatting	JSX, block components, indentation, edge cases
`idempotency.test.ts`	Stability	`format(format(x)) === format(x)` for all inputs
`html-blocks.test.ts`	HTML formatting	HTML block indentation in MDX
`load-config.test.ts`	Configuration	`.mdx-formatter.json` loading and merging
`url-autolink.test.ts`	URL handling	Autolink behavior and URL preservation
`rust-formatter.test.ts`	Additional tests	Extra Rust engine coverage
`rust-passthrough.test.ts`	Behavior validation	85 formatting behavior tests

All tests use vitest and can be run with:

pnpm test           # Run all 207 tests once
pnpm test:watch     # Watch mode
pnpm test:coverage  # Coverage report

Rust Test Suite

The Rust tests in crates/mdx-formatter-core/ validate the formatting engine independently. There are 342 tests across multiple test files.

Test Files

File	Count	Focus
Inline unit tests (formatter, html_formatter, config, parser)	124	Unit tests for all formatting rules, YAML, settings, config, HTML
`tests/cross_platform.rs`	165	Cross-platform validation
`tests/plugin_validation.rs`	42	Validates that original TS plugins are not needed
`tests/spacing_recursion.rs`	11	Spacing at all AST depths (blockquotes, JSX)

cargo test               # Run all 342 Rust tests
cargo test --test cross_platform  # Just cross-platform tests

How the Test Bridge Works

The napi-rs bridge enables running the TypeScript test suite against the Rust formatter:

vitest
  ↓
src/rust-formatter.ts    ← JS wrapper (loads .node binary)
  ↓
napi-rs binding          ← platform-specific native module
  ↓
mdx-formatter-core       ← Rust formatting engine
  ↓
formatted output         ← returned to vitest for assertion

Idempotency Testing

A core property of the formatter is idempotency: formatting an already-formatted file must produce identical output. The convergence loop (max 3 iterations) in the Rust engine guarantees this:

const first = await format(input);
const second = await format(first);
expect(first).toBe(second);

Testing New Rules

When adding a new formatting rule:

Write tests in test/ — these define the contract
Implement the rule in Rust (crates/mdx-formatter-core/src/formatter.rs)
Build the napi module: pnpm build:rust
Verify tests pass: pnpm test
Add Rust-level tests in crates/mdx-formatter-core/tests/
Verify Rust tests pass: cargo test